more of freerunning and tricking than parkour but i hate making picky differences. Anyways this may help you out a lot.
http://dogentricks.com/videos/DogenTricks.com%20-%20Gainer%20Tutorial.wmvalso check out
dogentricks.com in general. lots of good stuff that will greatly help you out with your height and control of flips. In particular look at the center of gravity and the blocking tutorials. You seem to have a pretty good sense of what your doing though.
I also suggest you try finding an open gym in your area and practicing flips there before you try it on the grass. makes things much easier lol.