Tease the living fuck out of her before you actually start. When you go down on her, don't dive right in. Do something like lick the area around her vagina, lick/kiss/bite her thighs, come back up and kiss the area between her pelvis and her belly button, lightly brush your fingers over her outer lips, things like that. Throw in a single flick of the tongue on her clit or a single slow lick from bottom to top while you're doing those things and then back off again. There's something about anticipation that drives women absolutely crazy. You'll know you're doing it right when she can't stand it anymore, grabs your head, and buries it between her legs.