I don't jump, so sorry for invading your section but..
If you sit crooked either your horse is crooked (hi-low,) your saddle doesn't fit correctly, or you are out of whack (one leg shorter than the other, tight hip, etc.)
Good trims will help the hi-low, if your saddle doesn't fit, get a new one, and go to the chiropractor to see if they can help you with your problem. Stretch before you ride, maybe take up some dancing to loosen you up more so.
Also look at YOUR diet. You need vitamins and minerals just as much as the horse does. Your joints need to be lubricated properly so make sure you are getting a good diet or at least supplementing those into your diet.
Best of luck!
"Strength is the ability to use a muscle without tension"