The female orgasm is just as individual as girls themselves. There are often times common threads among them, but in the end, it's always going to be different. All of the posted oral advice has been good so far, but the one thing I would add is definitely her mind. A previous poster noted his girl not being able to "let go." That is so true. If she is feeling any kind of pressure at all to have an orgasm, I can almost guarantee she won't have one. Do you guys talk about it a lot? Are you always pressuring her to "come on baby, just cum for me," blah blah. If you are, then STOP, otherwise she'll be cumless with you until she meets some other dude who DOESN'T pressure her, she's able to relax and "let go," and she cums like a porn star.
Some chicks also need to have a deep emotional connection to cum (while others can cum from an intense stare from across the room from a stranger). Do you make her feel confident about her body? Does she feel hot and sexy when she's with you or does she have low self-esteem?
There are so many layers to this question, it could go on for days. The best physical advice is to concentrate on going down on her. Keep the rhythm even while you're doing it too. If you're licking her clit, keep whatever pace you're going at. Once she's into it, don't speed up because you think it'll make her cum (unless she tells you you to). Don't do it harder, faster, or slower, or change direction with your tongue, or whatever. Just whatever happens, keep the same pace.
The best emotional advice is to just chill the fuck out, help her chill the fuck out, and don't pressure her to cum. Hope that helps.