As you rise out of the saddle (at a posting trot) look at the horses shoulders. You want to rise out of the saddle with your horses outside leg. If you are on the right lead, you will notice that as you rise out of the saddle, the horses outside shoulder goes forward. That also means that as you rise out of the saddle, the horses outside leg is infront of the inside leg.
If you switch leads across the arena, in the middle at 'x' or at the opposite end of the arena, sit one beat on the horse, then start posting the trot again. As you sit that one beat, you wait that one second for the horses outside shoulder to be infront again. http://forums.arabianbreeders.net/up...5852_thumb.jpg
If you hit the picture link, it shows a horses trot. If you can see, the outside leg is infront of the inside leg in the picture. That is when you rise out of the saddle, and sit the beat when the horses inside leg is infront.
I hope this makes sense. Try watching some YoutTube videos on it. There are some helpful ones out there.