In addition to your seat, you need to have your leg and your upper body in such a balance that if your horse was taken away, you would land on your feet. If you are a bit behind the balance point with your legs out front, you will fall on your bottom. If you are a bit too forward or your legs are too far back, you will land on your nose.
You also need to pay special attention to the angles of your elbows and knees. Think of having the weight of the rein in your elbow rather than in your fingers. Then your hand can be soft, supple and relaxed even when you are saying, "No, I'm not going to give you a longer rein." Many riders don't keep an angle at the elbow--they ride with straight arms so the horse meets resistance at the rider's shoulder, which makes the balance unstable. When the horse meets resistance from a deep elbow near the rider's center, the rider can stay better balanced. The elbow has to be deep with a straight line from the horse's mouth to the elbow.
As for the knee, many riders think that a longer stirrup leather makes a better dressage rider. However, when the leathers are too long, you end up with a leg that is too straight, and you can't sit around your horse anymore. In principle, because the horse's barrel is round, you can't sit with lower leg contact without bending the knee. You can find the correct angle of the knee if you stand on the ground with your feet the width of your horse and bend your knees. As a general rule, you want to bend your knees so that the top of your knees are in line with the tip of the toes.
A rider secure in his own balance will help a horse's balance and not be negatively influenced by it. I often think of how the late Herbert Rehbein sat on a horse. He would never crumble when a horse tried to get his own way. Rehbein was so strong in his position that he could keep himself balanced even if the horse tried to pull him out of balance. Ultimately, the horse would become more balanced himself because Rehbein maintained his balance.
Rehbein wouldn't move, so some considered him a strong rider. But the word "strong" when referring to an influential rider is misleading because it implies that tension is involved--and a tense muscle always has less feeling. I prefer to use the words "secure" or "centered" to describe the rider's position when a horse tries to push him out of place. I am a solid rider, but I don't need strong aids because I have body control that helps my position stay secure and centered. If the horse gets strong and tries to displace me, he usually is not successful. He then matches his balance to mine.
The balanced, centered rider is like the longeing girth and side reins. If a horse with a longeing girth wants to be longer in his frame, the side reins naturally resist him. But when he accepts the bit in the right frame, the side reins don't pull back. When the rider is like the longeing girth, he can resist the horse who is out of balance without becoming tense anywhere and without pulling back on the reins, stiffening his back or holding with the leg. He can stay secure and centered in his balance.
Checking Your Brake & Accelerator
Once your balance and position are solid, you can begin to influence your horse properly, but before you are able to determine the length of your horse's step, you need to confirm the ABCs or the basics of training dressage horses. You need to be able to stop your horse from your rein aid and make him go from your leg. This will allow you to make effective half halts--the key to asking your horse to take shorter steps and become balanced.
Stopping your horse from the rein is like using the brake in your car. When you use the brake to stop your car, it will remain stopped until you press on the gas. Once you have used the reins to ask your horse to stop, he should stand in place. If your horse doesn't stop from your rein aid, then he isn't going to be able to take shorter steps.