How should I contact someone for letters of recommendation?

I want to contact someone for a letter of recommendation. I don't see her anymore, I haven't seen her since august, but she said if I ever needed a letter, to ask her. Should I call her or email her? And when I do, how should I phrase? I want to be as nice as possible about this, because I may need to get two from for these two places I want to go to and are asking for these letters.|||A personal contact is best so call her./