start at the top of your leg to fix the issue at the bottom :)
Concentrate on your thigh. You want the inner part of your thigh (think where your thighs rub together when you walk) to be touching the horse/saddle, not the back of your thigh. If the back of your thigh is touching, this rotates your knee out, which also will in turn rotate your ankle out and makes your toes turn out too.
Think of sitting on three 'points' of a triangle in the saddle - your seat bones and your pelvic bone. Sitting this way will help 'open' your hip and allow the proper area of your thigh to make contact with the horse. Also, periodically throughout your ride, halt and take your hand and actually pull your thigh out and away from your horse at the back of your thigh to help you lay the proper area of your thigh on the horse/saddle.
It isn't easy, and is a mental and physical workout to get to the point where you don't constantly have to think of how you are riding, but it works.
Another thing you may be doing is gripping with the back of your calf (this can happen in conjunction with what I mentioned above). Once you have that thigh muscle being used properly, you will find you can take your calf off your horse and not feel unbalanced. I'm working with my bf right now to teach him to use his thigh instead of back of his calf to hold himself on the horse, and it is slowly helping his toes out issue. There is already the muscle memory there, so it is not going to happen overnight.
Last edited by EdmontonHorseGal; 11-16-2013 at 06:37 PM.