Everyone else has made some good suggestions. I would also be hyper aware of your body position as well, if you are leaning more to one side or the other, the horse has to compensate for the rider's unbalance. People have one side stronger than the other, and usually put slightly more weight in one stirrup than the other. Check to see if your stirrups are equal lengths as well, many people "feel" equal, but aren't. It's tough to determine by yourself what exactly you're doing up there, because what feels balanced and right to us as the rider may be very far from actually being balanced! Eyes on the ground are a HUGE help in this case.
I see one example of this all the time, a lady I know has a horse she can NOT keep on the rail when she rides to the left. She tried everything, all sorts of circles and exercises, kicking, spurs, a whip, nothing helped. Until she got some eyes on the ground that noticed she basically puts all of her weight in the left stirrup the entire time she was on the horse, and she had NO idea she was doing it. So the horse would be mostly fine at the walk the majority of the time, but at the trot or faster was constantly bent to the right because she was trying to get him out on the rail, but falling to the inside, because to the horse, it felt like the rider was basically standing in only one stirrup, so he was forever trying to get back under her. This is obviously an extreme case, but we all naturally have some imbalances, and they sometimes can affect the horses a lot more than we could imagine. I see a lot of riders that unconsciously put more weight in their inside stirrup, I was one of them myself! The horse can not bend or relax or move forward freely if they're constantly having to "catch" the rider who's leaning up there, or compensate for the unbalance. Still I have to almost always think about stepping harder into my outside stirrup for me to be balanced, and it was amazing how that one little thing makes a big difference! As soon as I do that my mare drops her head and moves her shoulders to the outside and bends properly and really fills out that outside rein like she's supposed to.
A good thing to experiment with is changing the weight distribution when you're riding, step slightly harder in each stirrup while riding around or turning, put more weight on one seatbone at a time, see if that changes anything with how the horse moves. If you move a certain way and all of a sudden the horse can easily do whatever it found difficult before, you may have found a clue!
Just something to be aware of:)
I also like to do lots of lateral work, shoulder in, leg yield, turn on the forehand/haunches, all that should help as well. If they ever fall in while riding, you just do a mini leg yield and put them back out on the rail, regardless of the gait you are in.
Last edited by albertaeventer; 08-14-2013 at 04:48 PM.