I agree with Brenda and Equestrian Life, I would add that you can try standing in your 2 points position with shortened stirrups to lower your leg. Another good exercise is to do raising trot without stirrups.
You could also try doing squats and other leg exercises with a medicine ball or other weight held between your knees/calves to help develop inner leg muscles.
Like equestrian life said, try standing in two point at all gaits. I also find siting for one beat and rising for three is really good because you have to make sure you use your muscles to lower yourself so you don't fall on the horses back.
Squats, calf raises, balancing on a stair/step with balls of your feet (like a stirrup). Hope this helps!
Work without stirrups for sure! Two point also helps not only your leg but balance! Bareback could be an option as well!