Virtually all newcomers to electronic cigarettes are smokers , so it is natural for them to try to use an e-cigarette in the same way as a tobacco cigarette, especially as the first model chosen may be a replica of a cigarette.
Unfortunately this doesn't work well, as the two things may look similar but are functionally very different. One is a paper tube where the smoke from burning chopped-up vegetable matter is inhaled; the other is a steel tube producing a water-based vapor from a liquid that is vaporized by a battery and heater element - essentially, a low-temperature steam. One contains tobacco that is set on fire at high temperature; the other has no tobacco and generates mist using a battery. There aren't really any valid comparisons between the two.
The most accurate term for an 'e-cigarette' is probably an EV, electronic (or electric) vaporiser. The vast majority of EVs do not look anything like a cigarette, although the sales figures are opposite to this: minis that look like cigarettes do have huge sales volume as they are by far the most popular with smokers trying out the concept. In addition there is no functional similarity between an EV and a tobacco cigarette. It is probably best to think of the terms 'electronic cigarette' and 'e-cigarette' as marketing terms since most EVs don't look anything like a cigarette and none work anything like a cigarette. However, hardly any smokers would switch if the name EV or something similar like PV or vaporiser was used exclusively - so 'e-cigarette' works very well indeed to get smokers interested. This tends to obscure the fact that the two things are different and are used differently. Mini ecigs, especially, need a different technique in order to derive maximum benefit with minimum problems.
If an ecig is used like a cigarette, it doesn't work, and there might be other issues too, such as a sore throat or lungs. This is generally applicable to a new ecig user with a mini ecig although it may not apply in other circumstances (expert users, with better hardware, make their own rules - this info is mainly for newcomers using minis).
An ecig doesn't contain any tobacco, there is no combustion, there is no ignition, there is no smoke; the process involves generation of low-temperature 'steam' from within a saturated atmosphere in an electrical device - not the burning of dry plant material. Multiple clinical trials have demonstrated that if an ecig is used in the same way as a tobacco cigarette, by beginners using a mini ecig, either zero or very little nicotine can be measured in the user's bloodstream.
So now that we know it must be used differently, the question is: how? The short answer is that the only similarity is that both are placed in the mouth. After that, everything else is different, when talking specifically about optimal use for new users using low-power hardware.
This information is specifically for new ecig users
There are no rules of any kind that apply to experienced vapers
This article should be read in conjunction with the one at: Inhalation and Vapour Ingredient Myths - because each clarifies the other.
If you are a new ecig user, and you have one of the smaller devices (especially one of the mini / cigalike models), then this applies to you:
1. E-Cigarette vapor is not drawn directly into the lungs, as many smokers do with a tobacco cigarette. The lungs do not 'suck' on the ecig; a direct inhale is not used. Instead, the vapor is first drawn into the mouth, using the cheeks to create a vacuum, then inhaled if desired.
The vapor doesn't have to be inhaled though, and an ecig can be used like a cigar, with the vapor only taken into the mouth. Nicotine will still be delivered as the water-based vapor is absorbed in the mouth (and nose).
2. First the vapor is drawn into the mouth (not the lungs), then held there for a second or two. Then, it can be inhaled if desired. The vapor is then expelled through the mouth or nose.
3. The vapor is drawn very slowly and gently into the mouth. A hard pull, as is normal with a cigarette, cannot be used since:
a. The heater coil doesn't work properly unless air is drawn across it gently for several seconds. There will be little or no nicotine in the vapor if a short hard pull is used.
b. A hard draw may pull liquid into the mouth.
4. E-Cigarette vapor is a water-based mist and not smoke. This has two main consequences:
a. The droplet size in vapor is much larger than the particles in smoke, and therefore it does not penetrate as deeply into the lungs. Vapor aerosol is ten times the particulate size in smoke (10 microns vs 1 micron). Vapor cannot go anywhere near as deep, and this means that nicotine is not available so easily from the lungs, and is obtained more slowly.
b. It is likely that just as much nicotine is absorbed by the mucous membranes in the mouth and nose as within the lungs. In fact for maximum nicotine delivery, all three need to be employed: mouth, lungs, nose - otherwise all the nicotine in the vapor will not be extracted.
5. Indeed it is not necessary to inhale, when using an e-cigarette, to obtain nicotine: holding the vapor in the mouth will suffice for some people (especially with high-strength liquid), and expelling some through the nose as well, after a very shallow inhale, will add to the nic delivery. Persons with serious lung diseases caused by smoking, such as emphysema, can take advantage of this - since inhaling anything further, even mist, is absolutely not advised.
Nicotine delivery via cigarette smoke is unusually rapid - it reaches the brain in around 8 seconds, which is at least three times faster than injecting nicotine, which takes up to 30 seconds to reach the brain. This is part of the grip cigarettes have, since this ultra-fast delivery becomes associated with the act of smoking.
Nevertheless, a properly set up e-cigarette, with a sufficiently strong liquid for the equipment and user, used correctly after advice from an expert, will deliver nicotine within 30 seconds. After a period of adapting to the new delivery method, this has proven acceptable to smokers switching to the ecig. Experienced vapers learn how to optimize this extraction process and that includes the speed of delivery.
6. Lastly, an 'inhale' on an ecig (which as we have seen above is nothing of the sort, it's more of a 'suck') is much, much longer than with a cigarette. On average, there is only somewhere between 10% and 50% of the nicotine in ecig vapor compared to cigarette smoke, so more vapor needs to be taken in, and for a longer time session - probably about 12 minutes as against the 5 minutes a cigarette lasts. Also, the atomizer simply doesn't work if a 2-second inhale is used, since for at least the first second it is warming up and does absolutely nothing.
With a small ecig we draw into the mouth for between 4 seconds and 8 seconds (yes, 8 seconds), depending on how efficient the device is, how strong the liquid refill is, and how expert the user is; the average draw length is 4 or 5 seconds. This would be used with a mini ecig, for example. As you can see, a slow and gentle 'suck' of 5 seconds bears no relation at all to the sharp, hard lung inhale of 1 or 2 seconds often used with a cigarette.
With a larger and more efficient ecig we might use a 3 or 4 second draw. We don't use a direct lung inhale or a 2-second draw with small ecigs, except by personal preference, for occasional variation. Neither works: a lung inhale doesn't get enough nicotine and causes soreness and coughing, and a 2-second cigarette-style draw is basically a placebo operation as the atomizer won't be doing much.
Please note that expert users, using larger and more powerful hardware, use a variety of inhalation methods.
Results of incorrect use
As we can see, incorrect use doesn't work and has issues: little or no nicotine will be delivered; coughing or irritation to the throat or lungs may result. An e-cigarette (especially a mini ecig) is not used like a tobacco cigarette for multiple reasons. Tobacco smoke contains materials that are engineered to have an anaesthetic effect on the throat and lungs, but e-cigarette mist does not have these compounds.
There are probably more clinical studies carried out and published where the researchers didn't know how to use an ecig than studies performed correctly with advice from an expert. We can easily see this in results that include reports such as '"zero or little nicotine was delivered" or "the lungs were affected". It is a little worrying to see such results since it brings into question the results of other studies: if clinical researchers are not competent to obtain correct results (by asking for advice from experts), could other results they obtain also be wildly inaccurate?
The sub-optimal results many researchers have obtained resulted from the assumption that EVs and cigarettes work in a similar way and are used in a similar way. Don't make the same mistake or you will also get poor results.
Always use your ecig in a different way to a cigarette (apart from them both going in the mouth). If you find yourself doing something - anything - the same, it's probably wrong and almost certainly suboptimal.
Experienced users, using a variety of different hardware and refills, use different approaches. Some use a direct lung inhale, sometimes where this is combined with a large airhole in an RBA. Because there is no one-size-fits-all solution in vaping, there are a multitude of different inhalation techniques. Read this thread for a description of some of them:
Lung Inhale vs Mouth Inhale
The posts in that thread are a clear indication that smokers, and vapers, use a wide variety of inhalation methods. The advice above is designed to allow newcomers with less than optimal set-ups to get the best results.
Definitely to be avoided
Try not to apply a lighter to the end of your e-cigarette.
Don't try to flick ash off the end, you'll look stupid.
Do not throw your ecig out of the car window after finishing with it.
This information is derived from multiple expert sources including clinical trials and lab tests that we know were executed correctly, such as by Dr Laugesen ; and expert vapers' experiences with multiple hardware and liquid types.
There is a wide variation between the effects of e-cigarette use for different individuals, and in some cases this has been demonstrated first here at ECF. For example, we can show that there is a factor 10 difference in individual tolerance to nicotine: some cannot over-vape 6mg (0.6%) strength liquid; some must use 60mg (6%) strength in order to avoid relapse to smoking. Because of this it is impossible to state what any given individual's response to a specific item of hardware, or e-liquid type or strength - and especially a combination of the two - will be. The differing effects on individuals are multiplied by equipment variations and then multiplied again by refill variants. Therefore, only the individual's own experience is valid, since the variables are impossible to calculate. There is no one-size-fits-all in vaping.
Because of this, no hard and fast rules can be stated. The above advice is a general guide, and best used as a starting point for individual testing and trial. It is specifically written for beginners and those not familiar with optimal e-cigarette use; it is especially applicable to mini ecig use. Experts find their own way of utilising their hardware-refill combo. With larger and more powerful equipment and/or strong refills, the inhalation method is less critical for deriving full benefit with minimum negatives.
 Current indications are that about 1 in 3,000 e-cigarette users (0.03%) were not previously smokers.
Etter, Bullen 2011
Equally, children don't use ecigs: ASH UK
 Dr Laugesen's tests for Ruyan are interesting, and can be found on his website NealthNZ.com. For example he showed that the nicotine content of vapor is one-tenth that in cigarette smoke, for the equipment and refill liquid combination he tested. However this almost certainly does not apply to other set-ups. The Goniewicz series of lab tests can also be referred to, which show that on average 50% of the nicotine in e-liquid is transferred into the vapour, with a range from 10% to 80% depending on efficiency.
There are three separate clinical trials demonstrating that e-cigarettes deliver little or no nicotine to the end user, as measured by a blood plasma test. These can be found by searching in the medical literature for the work of Vansickel, Eissenberg, and Bullen, in and around 2010. However there are major issues with these tests since none of them represent any other situation except the worst possible arrangement: beginners, given the wrong advice or no advice, isolated from expert advice, using the worst-performing equipment possible, using low-strength refills, and using incorrect technique that guaranteed a poor result. In other words, using a mini ecig; using a mini ecig that is not known for good performance; not told how to inhale; using a low-strength nicotine refill when we know that beginners may need a very high strength in order to counter the effect of poor equipment and poor technique (unless a placebo effect alone is required) ; and not told how to inhale correctly with a low-performance mini ecig.
When this was pointed out to Dr Eissenberg (who is a member here), he went back and retested using more positive arrangements, and got different results. He then re-published stating that e-cigarettes do supply nicotine.
So we know that some arrangements don't work. The advice in the article above is for the purpose of achieving the best possible result from equipment that may be sub-par. Equipment that works well such as a VV APV used with high-strength refill liquid is far less problematic, and for this type of arrangement it is more difficult to get a bad result than a good one. Therefore the advice is most needed by new users of mini ecigs using refills that may be far too low in strength for the combination of equipment and ecig-naive user. We now know that, for some equipment at least, 45mg strength e-liquid is needed in order to directly replicate a tobacco cigarette's nicotine delivery .
 This why Intellicig introduced a new 45mg (4.5%) retail strength of liquid and may receive a medical license in the UK for it: low strengths often don't work for beginners using suboptimal equipment, and they are shown to receive little or no nicotine as a result. They did the clinical testing required (unlike other manufacturers/vendors) and modified their products as a result of factual data. In many cases, low-performance mini ecigs used by beginners may need high-strength liquid (or even ultra high strength) to produce a usable result.
Like everything else, this isn't a rule: people are too different to make rules for in this area. The existence of a demonstrated factor 10 difference in tolerance to nicotine should make this obvious: people differ by one order of magnitude from each other (or more), and in such situations rules are worthless. A one-size-fits-all approach is a failure in just about anything to do with e-cigarettes, and this is one of the most important things to appreciate.