Regardless of which discipline you ride, it's very important to sit straight and square in the saddle. Can you tell if you're collapsing at your waist and sitting crookedly?
Ask a ground person to stand behind you.
1. Are your shoulders level (i.e. the same height)?
2. Is your seat in the center of the saddle so that each seatbone is the same distance from the middle of the saddle?
If your shoulders aren't level which means that one shoulder is lower than the other, you're collapsed at your waist, your shoulders aren't the same height, and your seat isn't in the middle of the saddle.
Here are 3 simple tips to help you sit straight and square:
Let's say you've collapsed the left side of your waist, your left shoulder is lower, and your seat is off to the right.
1. Bring your right seat bone over and place it on top of an imaginary line that runs down the center of the saddle from the pommel to the cantle. You won't really be on top of that line. You'll just end up sitting in squarely.
2. Stretch your left arm straight up so it passes by your ear and your fingers are pointed straight toward the sky. Feel how that elongates your left side.
3. Now to keep yourself straight and stretching tall as you ride, pretend you have two sticks of equal length between your last rib and your waistband. If you collapse again, you'll get jabbed in the ribs by the stick on the left side. And the stick on the right side will end up on the ground.
A happy Horse