All you have to do is practice. There really is not much else to say that will help. Flipping over your bars isn't really a bad thing. It means you are leaning forward enough, you just gotta be able to stay back more.
When I was learning nose manuals, I would set a goal to reach by the end of the sesh. Use chalk or just remember some cracks in the ground. Go out and don't stop until you meet that goal. The next day, (or later that day), make the marks bigger. Eventually nose manuals will be a cinch, and you'll be able to hold them as long as possible.