My advice: if you're doing it simply to "return the favor" then don't bother.
There's nothing worse than sex being twisted into some bartering tit-for-tat, "you sucked me, now I suppose I have to lick you" bullshit. I go down on a woman because I love doing it, because I want to taste her, to feel her smooth thighs against my cheeks and her warm juices smeared across my face and dribbling down my chin. I do it because I'm fucking ravenous for it, not because I'm expecting a blowjob in return or I'm paying dues for a blowjob I've already received. And I do it for me, because it turns me on and satisfies my sexual desire. If it happens to bring her pleasure, that's a fortuitous bonus and it speaks to our sexual chemistry.
Of course, if she doesn't like it or, more specifically, if she doesn't want me to do it, then I'm not about to force her to endure something she hates or foolishly attempt to change her mind on the matter, either by convincing myself she just hasn't had it done properly on her or that I can somehow learn a special technique that will make it pleasurable for her. In that instance, we're probably not sexually compatible and we should just move on.
Anyway, my advice is to only go down on a woman if you want to, if you enjoy it and she's open to it, and not as reparations for sexual pleasures rendered. As far as technique and skill are concerned, there's no single trick that applies across the board. Just approach it with enthusiasm while paying attention to telltale cues, to the way her body responses and the sounds she makes. And never be afraid to try new things. The exploration and discovery are half the fun.
