Ride a ReAlLy ReAlLy ReAlLy bouncy horse you'll be a natural. ;) Also your heels are probably your biggest problem, your trying to perch on your toe to push your self out of the saddle, not going to work. Have someone show you the two point and keep doing it until you can't anymore (probably won't take tooo long). Then break, practice again. Do this at the walk then move onto the trot. Then practice the two point without using the crest for balance, repeat previous steps over and over. Being able to get in and out of a two point postion is very similar to posting, its just doing it over and over again to the rythm of the trot. Once you truly sink into your heel and realize your foot won't fall out of the stirrup even with all your weight in your heel, you will find it much easier to post. If I were trying to post by standing on my toe I doubt I could do it, and probably not for long, I would think your using different muscles to do that then the ones your supposed to, will have to try it though to find out. Anywho, borrowing the bounciest horse in the barn would be my first guess at getting you to snap into it.