My first dog was a fence jumper, and a behaviourist told me it was because he'd spent so much of his life in the pound--confined in a tiny space, the only way he'd had to burn off energy was to jump really high!
We had success with walking around the fence, as recommended by another responder. He could clear a 6' fence, and often _climbed_ the fence if necessary.
We also noticed that he tended to jump the fence more at particular times of day--such as when our neighbors walked their dogs! The solution was simply to keep him out of the yard at that time. | 
Dog jumping the fence. 
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