alright, going to be as comprehensive as I can but I'm sure I'll leave somethings out, bare with me here.
first things first, "eating pussy" isn't just about jamming your face into her vagina and having your way at it. timing, sensitivity, and creativity are good things to use down below.
when you're eating a girl out, be sure that she's good and ready, get her worked up, play around with all of her erogenous zones before you just dive on in. Its good to know what she likes and is comfortable with, nothing but practice will help you with that.
once you've gotten her worked up don't forget that she's capable of being stimulated other places about her body. tease her up and continue to tease her about. don't just play with her pussy until you or she has finished.
now, having mentioned teasing, don't tease her when she's about to climax, once you've worked her this far don't back off or she'll be likely to lose that release and become frustrated and bored. As well, teasing isn't meant to be a wennie move, don't be a pussy mulling over her too gingerly. I've known many women that get turned off by a man who isn't firm and commited.
be sure to work her with not only your lips and tounge but use your fingers as well especially if your getting tired and need to keep things going. patience and anticipation are so huge, don't get frustrated or bored, genuinely enjoy what you're doing or she's going to sense your mood and start to shut down.
a good way to start is by getting her turned on, nibble at her neck, try a little nipple play, tenderly caress her whole body and get her really worked up. move on down south starting with your hands and give her a little tease, her anticipation should skyrocket as you're nibbling at her thighs and mons. when you're both good and ready go ahead and get in there. give her a little kiss, a lick or nibble whatever she seems to enjoy.
pressure placed on her mons can be very stimulating to some women, if you're tonging her place a firm hand on her abdomen above her clit.
do enjoy every last bit of her vulva and don't forget that there are other sensitive bits about her besides that clit, work her up but save singular focus there for when you're about to finish her.
depending on her, some women I've come to know enjoy the alphabet technique and some like a firm criss crossing of the clitoris and others like a little indirect stimulation from a tonge flittering about the sides of her hood.
a little bit of suction on her lips and clit can be a very stimulating act for some but be careful not to overexert as it can hurt to suck to hard.
don't forget a little penetration from either your tonge or your fingers and be sure to keep her comfortably wet, dry clit is never a good thing but there is such a thing as too sloppy.
there are a lot of other things to be mentioned but its all about getting out there (or should I say DOWN THERE) and trying things out. you know your partner better than any of us so just go with what feels right.