To achieve a great position all of the parts need to work together to properly balance you on the horse, while also creating a beautiful picture.
Strong Legs and Low Heels
Before riding get your leg position correct at the standstill. Have your stirrup leathers perpendicular to the ground with and toe and knee aligned. Grip evenly with the bottom of your thighs, knee, and top of your calves. Some riders have the bad habit of only griping with their knees, which causes them to pivot and tip forward in the saddle.
The two most important exercises for strengthening your leg and position are riding in two point and riding without stirrups. Practice your two point position as often as possible. Also, working without stirrups at all three gaits, in two point, and while posting. Remember as you practice to lower your heels as much as you can. Lower them each time you rise in the posting trot. Sink your weight into your heels and try not to post on your toes. At the canter lower the heels when the horse’s front legs hit the ground with each stride.
Two Point Position
Rise up into your two point through your heel. Lowering and weighting your heels stretches your calf and allows you to have more leg on a horse. If your seat is moving up and down in two point try lowering yourself in the saddle a bit more and driving your heels down. Some riders will “post” the canter. This isn’t only incorrect, but uncomfortable for the horse as the rider repeatedly hits him on the back. Most courses will be ridden in a half seat or a light seat which is a little bit deeper version. In the light seat it is important that you are not sitting on your seat, you are instead sinking even further into your heels. However, there are times where you will need to ride in a full seat, such as when your horse is spooky or you need to move the horse forward quickly.
Practice going into full seat, light seat, and two point at all three gaits. Also practice two point over cavaletti and low jumps. Land in your heel and keep your seat light in the tack. The jump from a two point position is only a fold at the hips and some sort of release with your hands. If you have to sit deep because you are deep to a fence or for whatever other reason, be sure to fold when your horse pushes off over the jump. You always want to go with the horse.