There are two issues to deal with concerning a leg that slips back in the air. First, you must practice exercises on the flat to strengthen and stabilize your leg position so that it never moves out of place. Riding without stirrups, particularly at the posting trot while keeping the leg in the same position that would be correct with stirrups, is a very important exercise. Trotting while maintaining two-point position, with the leg firmly in place, and practicing the sitting trot with or without stirrups are other useful exercises to strengthen and steady the rider’s legs. Also, working on maintaining the correct position while being longed is helpful.
Secondly, you need to think about the way you use your upper body over a fence–that is, let the horse jump up to you so that you only close your upper body as necessary to be in balance over your leg in the air, rather than leaning forward at take-off in anticipation of the jump, which will encourage your leg to slip backward. You can practice controlling your upper body by jumping gymnastic exercises, making sure that you don’t close your upper body any more than is necessary for you to keep your balance in the air.
Set four cavaletti poles on the ground with 4’6-4’9″ between them, according to the length of your horse’s stride. Then leave between 9′ and 9’6″ between the last pole and a little X jump, with the center of the X set about one foot off the ground. The next fence should be an oxer set at about 2 1/2 to 3 feet and about 2 1/2 to 3 feet wide. If you like, you can add two more oxers to the line, set at approximately the same height. The measurements between the three oxers (when approached from the X) should be about 18′, then 19′, and finally 20′. If you horse is short-strided, decrease these three distances by one foot each; and if your horse is very long-strided, increase these three distances by one foot each. (You can find detailed information and diagrams concerning gymnastic exercises in my book, The Complete Guide to Hunter Seat Training, Showing, and Judging, as well as in Bertalan de Nemethy’s book, The de Nemethy Method, which is currently out of print, but can be acquired as a used copy at numerous places online.)
In order to work on your upper body, approach the exercise in two-point position, so that the angulation of your upper body will keep you in balance above the angle of your lower leg. Don’t move your upper body forward as the horse nears the take-off spot to the first oxer, but allow the animal to jump up to you, so that its withers rise to close the space in front of you.
All you need to do is stay balanced over your own legs, for there is nothing you can do to aid your horse in jumping and much you can do to distract the attention and disturb the balance of the horse by moving your upper body forward in the air. By controlling the desire to throw your upper body forward at take-off, you’ll help yourself keep your leg in the proper position, adding more downward weight to the stirrup so that your lower leg won’t slide back so easily.