Don't be in a hurry to attack her clit. Kiss, nibble, and suck on her mouth, her breasts, her thighs. Like any sex act, she needs to be relaxed and ready before you start getting serious. Even when you do make it down there, kiss and lick and suck on all of her, not just her clit. Her labia need attention too, and many women love having a tongue pushed up inside of them.
2. Find the clit
You need to find the spot near or on her clit that is "just right" for her, the area that, with every flick of your tongue, sends surges of pleasure through her body. No, we're not going to draw you a picture, or describe a roadmap. The best way to find it is simply to ask her. Each woman is slightly different in how she wants to be stimulated, so tell her to give you guidance as your explore her nether regions.
Note: some women find direct clitoral stimulation too intense. You need to communicate to find the way to stimulate her best.
3. Relax your tongue
Many men tense up their tongues and flick the tip furiously against her clit. That's all wrong. A relaxed tongue will do the trick better, using the broad flat part, whether you flick it or just bob your head.
4. Use your hands
Your hands are free, so use them! A little finger-fucking is a great complement to clitoral stimulation. You can also caress her labia, lubed up with her own juices, of course. If you're an adventuresome couple, even try putting a finger or two up her ass. However, be careful not to put them anywhere near the vagina after that. A pair of latex gloves and water-based lube will definitely come in handy.
5. Get down to business
Once you've gotten her excited and figured out her hot spots, it's time to get down to business. Flick your tongue with a steady rhythm over her clit. Don't stop, and don't change the rhythm. That's the key to making her come: patience and a steady stroke. Don't vary things too much, and for goodness sake, don't ask her "did you come yet?" Let her fall naturally into an intense, mind-blowing orgasm