Being on the lungeline with your knees meeting at the top of the pommel and your hands resting on top. It seats the rider on their seatbones and helps them feel each step the horse takes. Also if you bring your legs back and up, then reach and hold the ankles, so you're in a backbend shape, that also seats you on your seatbones and trains your legs to stay back.
These are good balance exercises but they are tiring and can be a little nerve racking at first. Another good one is reach forward, touch the horse's ears without your legs going back. Reach down touch your toes, reach back and touch the horse's tail with your chin up and body relaxed.
"Strength is the ability to use a muscle without tension"