If you are tipping forward it is probably because you are allowing your lower leg to slip too far back. The lower leg is the counterbalance for your upper body. You MUST keep it forward at the girth to help counterbalance your upper body going forward. The further you go forward, the more forward it feels like your lower leg must be.
Actually, it is caused because of the rider pinching or gripping with their knee's. When they grip or pinch with their knees, the lower leg will naturally fling back, and the upper body will in turn, go the opposite direction.
Lilly, there are many great books out there that will help you mentally get what you want to do with your body. For one, I highly suggest you reading George Morris. He will really guide you and help you mentally see where your body should be, and what it should be doing.
I also suggest you start reading Practicle Horseman, where GM's critique columns are - you will obtain and learn a lot from reading what he has to say with indavidual pictures that are sent into him.
When you mentally see where your body should be, you can apply that when you are in the saddle.
GM also has fabulous DVD's/Video's as well.
As already stated, work a lot on 2 point. Finding your balance over your feet. You must be balanced over your feet.
The most important factor to remember is, your bodies weight must naturally be dispursed into your heels. Your heels must be allowed to be your anchors. Without them, you are not stabalized or solidified in your tack.
But in order for them to do their job, your body must allow the natural weight flow to occur. First, while at a stand, sitting in your tack, close your eyes and feel your weight. From your head, into your seat, and into your heels. Open your knees, you must learn to allow your knees to remain soft.
Secondly, you must find your proper leather length. If you find you are reaching for your toes, they are too long. Then you have to beable to place your foot in the iron correctly, or you wont beable to have a soft, flexable ankle - which in turn wont allow your heels to do their job.
All this - will be shown to you, through GM's teachings. I can go on and on and on about this, but I think this should be better left to GM himself :) Besides that, I don't have time - I have to get ready to head to work.