Don't plan anything, because if you do it will come off that way. How good are you at holding a conversation? If you're not very good, approach her while she's around people so other people can speak and there's less focus on you; however, the drawback to that is you better talk and outshine the others or else you'll be pretty forgettable. If you've met once before, I would say "Hey (her name), how've you been?" Then take it from there.
Always tell people exactly how you feel, at that exact moment. If the moment has passed, let us help you:
www.HopeYouGetThis.Com
L’esprit de l’escalier