For my students to maintain proper balance, I teach them the basic position of sitting in the center of the saddle so they are right over the middle of the horse. Both feet should be in the same position in the stirrups, with an equal amount of weight in each. This is the basic position for riding in a straight line, and in circles.
Correct posture is important.
A rider should sit tall, but relaxed, in order to maintain proper equitation. Your weight should be distributed so that you have 60 percent in your seat and 40 percent in your feet. Many riders have weak legs, and ride with almost all of their weight in their seat. This does not allow them the proper amount of leg control for balance and leg cues. These are riders who, during a stop, tend to fall forward and lose their balance point.