The key is the right speed and pressure throughout the entire area of the females genitals. Take it slow and gradual in order to build the anticipation. There is nothing like the warm, wet feeling of a lover's mouth caressing, flicking, and massaging the sensitive clitoris. The heat of your mouth, and the slick feeling of your tongue sliding around and over the vulva is a sensation like no other. Flick your tongue like a dance over the hood which covers the clit to tease and build the arousal while at the same time using your finger to circle lightly around the rim of her vigina. Treat it like a delicious mystery, or a delectable puzzle to solve. But do take it slow; your there for a sensual dinner not a quick snack.
Another suggestion I could make would be that you 'play dead', whereby she is in control and is doing all the work; you just keep your tongue held steady. She will then know what she likes and the time when she comes.