Lick at it while maintaining the suction. While direct clit licking is too intense for some, the 'circles and eights' swirl of your tongue looping along the sides, around the top, and under her clitoris can be exquisite. Build up the intensity level of sensation slowly, but once you're into it, use your whole face. Bury your self in her pussy as far as possible. If your eyelids aren't sticky, you're not doing it right.
Most women achieve an orgasm in response to rhythmic stimulation.
If you're in the groove, and you can tell by her breathing that she is building up to one, don't suddenly change what you're doing. There is a balance between variety and consistency that is key to becoming an oral expert. Find that balance. When you have finally brought her to orgasm with your mouth for the first time, your next move is so important it can nearly make or break the experience. A true orgasm will turn a woman into a single nerve, the whole of which is focused on one place.