I have long since pretty much given up on trying to post as a training tool. In western I don't post at all unless I'm trying to get the horse extend and stretch itself. After all, you really aren't supposed to post in western, are you?
On trail rides, endurance riding, cross country, I post "randomly", usually 2 up - 2 down to keep up my energy etc.
In english I usually post as a way to get the horse moving in a correct way -- much like in western.
If I'm riding a really slow and "sticky" horse, I post to get it moving better. If I'm riding bareback on a nervy horse, I post to get him/her think and slow down: posting randomly really throws most horses!
Are you trying to train the horse to trot slow/jog slow? If so, don't post, just sit deep and look down and close(r).
If you want to post and find it difficult to keep the correct rhythm, chances are that your horse isn't moving forward enough. If that's the trouble, try getting the horse in the fastest possible trot without leaving you behind: it doesn't matter if it's quick and short, on the contrary. Then, start to post slow and don't worry about getting the correct diagonal every time: it doesn't matter that much unless you're riding a lot of trot and on a circle. When you sit down, try to sit deep and heavy. If you sit two, it's okay. Rather sit 2, 3 or even 4 steps than be on your stirrups more than one: it slows the horse better.
You can also try posting two steps at a time, but I find that less effective than "sitting two standing one" when it comes to speed control.
But the best way to slow down your own pace? Sit deep when you sit; don't "stand" when you post: rather move your pelvis just slightly up and forward by crunching you butt muscles tighter; breathe; think; count slow one-two-one-two; don't think what the horse is doing with its legs; don't worry about keeping the correct diagonal; practice, practice, practice.
Last edited by kayhmk; 07-11-2010 at 08:02 PM.