The cheapest, and all natural way to get her off via her clit is to use your tongue, your mouth, and your hands. Rub her clit gently, nibble on it, lick it, see what turns her on while you're doing it and soon you'll be giving her a mind blowing clitoral orgasm. Persistence pays off in this case, as some woman take longer than others to have an orgasm, but the chance of success is high if you do it right.
So what if you've done everything you possibly can and she still doesn't have a clitoral orgasm? That's when you bring in the big guns! Vibrators, and especially high power vibrators like the Hitachi Magic Wand or the Miracle Massager, will induce a mind blowing orgasm in her quite quickly. Some women can barely handle 30 seconds with the Magic Wand before they start going insane!
If using your hand just isn't enough, there are attachments you can buy for the Magic Wand, as well as vibrators that are specifically designed for G-spot stimulation.
So what happens if none of these methods work and your lover just isn't capable of having an orgasm? Well, it is normal for a significant portion of the female population not to have an orgasm. Don't feel bad because you can't give her an orgasm, but instead feel great that you tried your best to give her one!