Dog people, I need your help.

[msincredible]

Once your dog learns to jump a fence it's pretty hard to train them out of it. It's very self-rewarding.

My whippets routinely jumped 6-ft fences (straight jump to the top) and Grover once jumped a 10-ft fence and broke multiple bones in both front legs/wrists coming down (he was being boarded, it was an unfortunate accident).

Grover had learned to jump them before I got him, Watson learned from him.

I agree with the physical fixes suggested above.
Another option for small dogs is this:
http://www.catfencein.com/