A basic balanced position is one where the ear - shoulder - hip - heel are all in alignment. This balanced position allows the rider to be more effective in the use of aids and have better control of gaits. Because the rider is balanced on the horse and with the horse's movement the two are able to have a relaxed partnership instead of impeding each other.
Legs should be relaxed and draping around the horse's barrel with the heels down, open at the hips with knees and toes pointed forward. To achieve this, try dropping your stirrups and stretch your leg out then back, then try to pick up your stirrups without moving your thighs forward
Your back should be straight with your core engaged. Try not to slouch forward or hollow your back. This will rotate your pelvis and make your seat less effective. Rotate you shoulders back and keep your eyes up and looking ahead of you.
There should be a straight line from your elbows to your wrist and down the reins to the bit. Elbows should be softly bent and hanging vertically from your shoulders. Reins can be shortened or lengthened to achieve the optimum contact with your horse while still maintaining good position. Hands should be soft and steady to avoid jerking on your horse's mouth.