The best method to use is a straightforward one. Ask her what she wants. If she seems resistant to the idea, you may want to try a dinner of libido increasing foods, such as red wine and oysters. You can even try hypnotism. If she's more open about it, then you can suggest a supplement or cream.
To choose the right supplement, make sure you look for these ingredients: ginkgo biloba, tribulus terrestris, hops extract, and horny goat weed. These herbs work together well to increase libido for most women. However, you may want to avoid them if your girl has blood pressure or heart problems.
Most creams will work by increasing blood flow to the area. Just make sure she doesn't have any allergies, or you may end up having a trip to the hospital rather than a romantic evening.
Hypnotism doesn't have to involve a pocket watch and "you are getting sleepy..." Instead, just train her mind to associate a certain external stimulus with pleasure. Scented candles work great for this. By having the same scent in the room every time you have sex, you'll eventually be able to get her aroused simply by lighting that candle.