Instructions
Don't be in a big hurry to start jumping. Before you can even think about working your horse over fences you need to spend several hours working with an instructor to develop and hone your basic equitation skills. Don't even think about jumping before your riding instructor tells you that you're ready.
Drop your irons. Rider instructors know that the best way to prevent a student's legs from slipping is to teach them how to ride without stirrups (also called irons). Riding without your stirrups forces you to wrap your lower legs around your horses barrel. Stirrup-less riding also teaches you to sit deeply in the saddle, to sit back and improves your overall balance.
Start with little fences. Once you have mastered riding on the flat without stirrups, your instructor should be ready to let you start jumping. Don't expect to start with 5-foot tall jumps that have a 4-foot spread. You have to develop your skill, timing and lower legs before you can start jumping the big fences.
Drop your stirrups. After you are comfortable with jumping small fences you will have to drop your stirrups and jump without them.
Put one arm behind your back. If you've become comfortable riding over fences without your stirrups and your lower leg is still slipping, you will have to pick up your stirrups and put both of your reins in one hand. Place your free hand behind your back. In addition to keeping your lower leg securely tucked underneath you, this exercise will also force you to look up.
How to Keep Legs From Sliding Back While Riding
Part of being an effective rider is having proper equitation. Equitation is the rider's form when they are mounted on their horse. When you are properly seated on your horse, there should be a straight line from your ankle bone to your hip. This is the proper placement for your leg for a variety of reasons. 1) It looks good. 2) It only takes a fraction of a second to move your leg back to signal your horse and guard the hindquarters. 3) When your lower leg is properly placed it increases your stability in the saddle. Having a secure lower leg is even more important when you're working your horse over jumps. If your leg slips backward when you are going over a fence, your upper body will rock too far forward and you run a risk of falling when your horse lands. In addition to throwing you off balance, there is a good chance that your horse will interpret your slipping leg as a signal to go faster and bolt.