I have a method that is a really a combination of two popular ones:
First I heat tap water as hot as possible and fill a cup with it. I use a pediatric syringe to pull the hot water up into the carto (after removing the plastic bits) and then force it back out. I then give it a few more forceful pumps of just air which
really gets out the old juice lingering in the filler.
Then I boil for 15 minutes. Repeat the above process and boil again. Finally, I use the syringe again giving it 40 or so pumps of just air to quickly dry it.
Presto chango, a denastified (I made a new word, I will file right next to my other original word, "automagically.") carto ready for juice.
(@Adrenalynn: Please feel free to rip apart my post, your snarky comments crack me up!)
Okay, so you are probably saying to yourself, "Self, that is a lot of work." Well you're right and really you shouldn't be talking to yourself in the first place.
The bottom line for me is that if I try X juice and discover I want to puke after 2 puffs, I don't really like the idea of trashing a brand new cartomizer, I'd rather just clean it and move on to another juice that hopefully isn't quite so bad.