Lead and follow. Connection. Balance.
Body position influences your horse whether you are standing still or moving. Correct posture is a basic skill riders must master in order to progress up the riding tree. Unless you correctly position your own body, you cannot expect your horse to use its body correctly.
What Its Supposed To Look Like
If you could view yourself from the side when sitting in the saddle, a vertical plumb line should go through the middle of your ear, through the middle of your shoulder, hip and heel.
If your horse were to disappear you should be in a standing position on the ground, knees bent.
As a rule, if you glance down and see your toe, your leg is too far forward. You are in a chair seat, tail bone tucked under and you will be behind your horse's motion.
Behind the motion causes your horse to either scoot forward or slow down.
Many riders tip forward to balance themselves trying to fix the problem and compromise their vertical alignment even more.
One knee (or both) should not 'creep' up the saddle. This can result in rider hip pain and rider crookedness.
Your horse should equally bend through his rib cage on both sides.
Your mid-section should be lifted and firm so that your pelvis and hips can move freely as well as absorb your horse's motion.
With a straight spine and toned abdominals, your pelvis can tip up and forward to set the rhythm at sitting trot without going faster than your horse. Riding from you core.
Your shoulders must stay directly over your hips in order for you to be balanced over your horse and able to follow his movements.
You should feel equal pressure on your seat bones. Sitting like you are on a sofa is not correct. You are putting too much pressure on the cantle of the saddle.
Conversely, riding too far forward causes your upper back and head to be too far in front of vertical. You are riding on the horse's forehand
Correct seat position allows for correct horse influence.
Your torso should stay perpendicular to the ground, not tip forward or backward, as you and your horse move rhythmically in balance.
Why is it so difficult to sit correctly?
Think about how you sit and stand everyday. This is your go-to place.
Why should sitting in a saddle be any different? Except the horse is moving.
TIPS TO HELP
If Your Ride Too Far Forward
Possibly your chest muscles are stronger than your back muscles causing your spine to round forward.
If you have too much arch in your lumbar area, possibly your hip flexors are too tight.
If You Ride Too Far Back
Possibly your abdominal and lumbar muscles are too weak to properly hold your torso upright.
You may be sitting on your tailbone putting too much weight on your horse's thoraco-lumbar area - on top of the kidneys. Your horse will be back sore.
Work on strengthening your core and glute muscles and practice sitting squarely on your seat bones.
It is often difficult to know if you are sitting crooked in the saddle. One of your shoulders may be higher than the other, or your torso may be rotated. This creates problems for your horse like difficulty getting the right lead, stiffness on one side while in a shoulder fore being unable to bend through his rib cage on one side.
On the ground, check your stirrup length. If one is longer than the other, you are riding crooked. Consider how much continuous load must be present on the longer stirrup leather, to stretch it out.