Learn to Relax
Rider position is crucial to maximizing your horses performance. The fundamental goal of the rider is to be relaxed and effective with the aids. Without the relaxation you will not be able to ask your horse to do anything productive and problems may arise from the tension.
The shoulders and arms hold the most tension for the rider. Some simple excercises are to constantly relax the shoulders and arm muscles, offering your hands toward the bit and discover how your horse reacts. Most likely, he will soften his neck muscles, round his back and be more comfortable.
Most importantly, breathe. When you hold your breath, you created tension in your body and thus in the horses. Remember, your horse is a reflexion of your energy. So relax, breathe, enjoy the ride.