If a rider is sitting crookedly in the saddle, then it is impossible for the horse to move straight and in balance.
To check if you are sitting evenly on your horse, ask someone to look at you from behind in halt (make sure your horse is standing square) and then in the walk. Are you collapsing more to one side or are you sitting squarely?
If you are alone and have access to an arena with mirrors, look at yourself as you ride towards them. Are your stirrups even or is one longer than the other? Are your shoulders level or is one dropping down?
Here are five tips to help a crooked rider sit straight:
1. Find your seat bones
With somebody holding our horse, take your feet out of the stirrups and bring both legs in front of the saddle, keeping your knees bent. Move around a little until you can feel you are sitting equally on both seat bones. Bring your legs back into position and retake the stirrups.
2. Stretching up
With your horse in halt, if you are sitting more to your right, place both reins into your right hand and stretch your left arm up, lengthening the left side of your body to correct your position. Continue the exercise in walk and see if you can feel a change in how you are sitting. Do the same with the right arm if you sit more to the left.
3. Look over your shoulder
If you put more weight on one seat bone, example the left, look over your left shoulder for a few strides and feel your weight automatically shift onto your right seat bone, and vice versa, helping you to sit even. This exercise is particularly useful riding on a circle or asking for a canter transition, as that is when the rider is more likely to collapse to one side.
4. Riding with one stirrup shorter
Shorten the stirrup of the side you are leaning on by about four or five holes and ride like this for a few minutes. Riding with one stirrup shorter encourages you to sit straighter, and lengthen the opposite leg, giving you the "feel" to sit correctly. Return to your regular length and see if you notice a difference.
5. Riding with one stirrup
Drop your left stirrup and ride on both reins in walk and trot, alternating between rising (posting) and sitting, on straight lines and circles. Do the same dropping the right stirrup. You only need to do this for a short period, but it will increase your awareness as to which is your weaker side, helping you to redistribute your weight in the saddle.
Having regular lessons with an experienced instructor will help rectify any crooked issues you may have and, if possible, ask someone to film you riding so you can see for yourself how you are sitting.
Pay attention when driving or sitting at your desk that you are sitting equally on both seat bones and do everyday tasks using your weaker side as well as your more dominant side. Taking classes in Yoga and Pilates are both excellent for improving your balance and body alignment and will benefit your riding.