The best advice I can give is to have her tell you what feels good and what doesn't and figure out the best way to please her. Every girl is going to like different speeds, different amounts of pressure, different patterns, etc. You have to be vocal about oral sex for it to be good, and I think the same thing can apply for women giving men head as well.
I like to start off with slow, light circular motions around my clit, and as I get worked up, I like the guy to slowly speed up and start to do faster and faster up and down strokes. Repeart that process two or three times and I'm there.