Join Date: Sep 2007
Location: Washington State
Given that this is a problem that seems to be more noticeable when you are posting the trot, make sure that you are not unconsciously using the reins to help you balance while posting. It's somewhat instinctive to want to use our hands for balance or to catch ourselves when we feel unsteady, but that's the last thing that you want to do while riding because your hands need to communicate through the reins and you certainly don't want to create unnecessary "interference".
The best thing would be to practice on the longe line with a school horse, posting the trot with no reins. As you get better, post with your arms held out to the sides, in front of you, over your head, etc. If you don't have someone to longe your steady-eddy horse, you can also try posting the trot with your horse on a really long loose rein so that you don't feel as tempted to try to "steady" yourself with the reins.