How do you ask for a kiss from a girl who is your friend?

Perhaps it makes no difference, but I would offer to give her a kiss, and not ask her for one. The difference is very subtle, but you are not asking her to do something. You're asking her to accept something from you. Just say "May I give you a kiss?" or "May I kiss you?" even "What would you say if I offered you a kiss?" This makes it even less direct and allows her to be in control. No worries if she declines. If you offer her a choice you MUST respect her desire. DON'T PUSH. The request alone works its magic over time. You'll see.