It might be glib, but...
http://crap.jinwicked.com/?comic=302
Legitmately, though, if you feel good enough about your prospects to ask, she'll probably say yes. If you don't, she probably won't. This is assuming you have some baseline attractiveness, but most people are looking in a partner for a friend that they can have sex with. If you're friends already, you're halfway there. A little confidence and not being the type of guy that bums her out with a constant stream of self-hate would probably go a long way.
One way or another, just don't be the stalker-in-training guy that drops constant unsubtle hints about how you're madly in love with her and tries to guilttrip or pressure her into a relationship. If you're thinking of expressing anything along the lines of, "I can't live without you", just don't.