Have you thought about investing in a pair of millbrook leathers? I know when I first got mine they helped me tremendously with keeping my heel down and my leg still. :) Also the suggestion for good stirrup irons is a good idea too!
I have the same problem with both my ankles. I've found that ankle strengthening exercises like Brenna suggested work really well, as well as ballet exercises. I did ballet for a few years and things like plies, tendus, degages, andother basic barre exercises really helped me. Additionally, if you need extra support, you can try wrapping your ankle. I've been wrapping my ankles for years.
Do you have the good stirrups to help you?