Well you could go the opposite route of everyone else...that is if you want to keep her guessing until the very end.
Just act depressed and really down whenever she is around. Eventually she probably will want to sit down and talk about you. And when she asks whats wrong you say something to the effect of: I have just been thinking about how I would feel if you werent in my life. How I would feel if you said "no".
And during that last sentence pull out the ring.
Good luck man. I hope everything turns out well with you and your GF.