Super Squats strengthen all the muscles surrounding your knee for improved knee strength and stability. You can also use a small pillow or rolled up hand towel.
Start standing with your feet about ten inches or more away from the wall, and your back against the wall. Make sure that your spine is long and take a deep breath here first. Place a 4" mini soccer ball (or a similar ball with a little give to it) between your knees and squeeze the ball with your knees while sliding your body down the wall until your thighs and lower legs are a right angle. Hold and squeeze for a few seconds and then slide back up the wall. Work up to 10 reps. Inhale deeply on the way down and exhale fully from your mouth on the way back up.
Note: If you can't get down to a right angle position at first, don't worry. Go as far as you can and practice every day. You'll get there!