I agree that all you need is an atomizer and a drip tip... but I like and recommend a drip shield, it keeps leaks from getting on your hands and battery. It doesn't seem that everyone feels the need for one, so maybe I'm just dripping an extra drop each time or something. Anyway I was getting juice leaking and tried a drip shield -- it solved all my issues.
If you do get a drip shield, you will most likely need a 510-510 connector. That will extend the length of the atomizer so that the drip shield can fit over the atomizer. If you get a sealed 510-510 connector, it can also help to protect your battery by sealing it off from leaks.
Another optional item would be an eGo cone, just purely for aesthetics. It makes your Twist look nice by covering the exposed eGo threading. If you really end up liking dripping it may be worth it to get those extra components.
Drip Shields only work really well with 901 atomizers. With 901 atomizers, the drip shield will run the extra juice straight back into the atomizer. with a 510 atomizer, the drip shield will help stop the mess on your hands, but it will still be there underneath the drip shield, meaning when you try to take the atomizer off, all the mess then goes all over when you are done. I don't see the point. I mistakenly bought drip shields for my 510 atomizers and it was a waste.