The first thing to realise when dogs jump over fences is that their behaviour is completely normal. The reason dogs jump over fences can be attributed to a variety of things, but for the most part, it's simply a case of curiosity and a desire to explore.
Exercise
Many behavioural traits that are perceived as negative in dogs arise due to a lack of activity. If your dog is worn out through exercise he or she is less likely to have the desire to jump over a fence. You should walk your dog regularly and incorporate some fun activities such as playing fetch with a ball or frisbee.
Positive reinforcement
Many people rely on punishment to train a dog. In reality, this doesn't work most of the time. The reason for this is a lack of long term memory. If your dog jumps over and fence and receives a punishment, he or she will unlikely recall why the punishment is being given.
As an alternative, it's better to reinforce desirable behaviour as a way to tell the dog that good things happen if he or she behave in a certain way. For example, if your dog goes into the garden and plays with a ball instead of jumping the fence, you should provide a treat. Over time, your dog will learn that certain behaviour leads to delicious treats.
Last resort
As a last resort, you could attempt to raise the height of your fences, or keep your dog on a lead when he or she is outside. I would suggest that you try the earlier recommendations before this though.