Im a TAEKWONDO instructor i have seen this problem with many students believe it or not i would recommed building up your calf muscles they will help absorb the shock.and give you greater balance too.

I would not only work on building the calf muscles but all muscles that support the ankle. That would mean exercises that force you up on your toes (calf muscles,) pronating the ankle out (with your foot flexed up, using a band on your foot), supanating the ankle in (with your foot flexed up, using a band on the foot by the toes), and flexing the foot up and down (pointed) again using a band.

There are many other different types of exercises you can do. Here are some links:

http://video.google.com/videosearch?q=videos+of+exercises+to+strengthen+the+ankle&rls=com.microsoft:en-us&oe=UTF-8&um=1&ie=UTF-8&ei=NCBESs3XFI7kNcnQyZ8B&sa=X&oi=video_result_group&ct=title&resnum=872198241#

http://video.google.com/videosearch?q=videos+of+exercises+to+strengthen+the+ankle&rls=com.microsoft:en-us&oe=UTF-8&um=1&ie=UTF-8&ei=NCBESs3XFI7kNcnQyZ8B&sa=X&oi=video_result_group&ct=title&resnum=872198241#

http://video.google.com/videosearch?q=videos+of+exercises+to+strengthen+the+ankle&rls=com.microsoft:en-us&oe=UTF-8&um=1&ie=UTF-8&ei=NCBESs3XFI7kNcnQyZ8B&sa=X&oi=video_result_group&ct=title&resnum=872198241#
(Crazy music in that one :-))

Kirk Mango
http://www.becomingatruechampion.com/
http://www.becomingatruechampion.blogspot.com/

Thanks guys!! This really helps! :]

In addition to strengthening your ankles make sure you are not landing short in your skills. Be sure to land with you feel under or in front of you in your tumbling. Landing with your feet behind you can lead to you crunching your ankles.