An exercise my trainer has me do is set up a 3 jump line, with however many strides in between but I would reccomend no more than 3 strides in between, and jump them with no reins at all. Tie the reins in a knot, and once you're headed toward the center of the jumps, let go. Focus on keeping your heels down and NOT gripping with your knee, because that makes your leg swing. Keep your lower leg on with pressure and your back straight from take off to landing and beyond. Try and stay in your two point without letting your bum come back and hit the saddle. If you can, do it without stirrups. It really helps not only your leg, but your position as a whole!