ankle mobility

I am one of the few unfortunate women whos ankle is not flexible enough to straighten out when in heels or when pointing my toes. It use to be so bad that I couldn't even sit on my feet. I have made progress and that is now possible but they still have a long way to go. Any advise on how to get a better range of motion? I feel like I am lacking strength in my feet since I cant get on my toes very high but there may be lack of flexibility in my toes as well.