I am copy-pasting this from another post 'cause its bed time and I am sure that you will receive very helpful responses in addition to this one! :) best of luck!!
"You're riding in a chair seat, which means instead of keeping your heels under you, they're out front and your seat is waaay back on the cantle. Try this: take your feet out of your stirrups, and (assuming your horse won't mind!) bring your knees up over the pommel/on either side of the horn. Feel the way that your butt is sitting in the saddle? You're kind of sitting on your seat pockets... keep that position and bring your legs back down (out of the stirrups). Feel the line from your shoulder, through your hip, down to your heel. You might have to lengthen your stirrups to fit this posture.
Another tip-- when you ride, think about lifting your chest (or sticking out your chest! Haha). This will help you set your shoulders back instead of tipping them forward."
ETA... fwiw, I know that some brands of western style endurance saddles tend to place the fenders forward, making a chair seat easier than a proper seat. I would check your saddle to make sure that isn't the case!
Last edited by existentialpony; 02-18-2013 at 04:13 AM.