How dogs escape and handy fence tips
Some dogs jump fences, but most actually climb them, using some part of the fence to push off from. A dog may also dig under the fence, chew through the fence, learn to open a gate or use any combination of these methods to get out of the yard. Knowing how your dog gets out will help you to modify your yard. But until you know why your dog wants to escape (see here), and you can decrease their motivation for doing so, the recommendations below won't be nearly as effective.
For climbing/jumping dogs:
Add an extension to your fence. It's not so important that the extension make the fence much higher, as long as it tilts inward at about a 45-degree angle. Be certain there are no structures placed near the fence, such as a table or chair or dog house, that your dog could use as a springboard to jump over the fence.
If your dog is jumping the fence - consider adding a height extension or a lean in top. Most dogs cannot jump (without climbing assistance) more than 6ft, so a fence this high should almost always be sufficient.
If your dog is jumping and climbing, you can add a simple SPINNING PVC pipe along the top edge of your exisiting fence so your dog cannot get a grip to climb over your fence. These are inexpensive, easy to install and remove options for people who are renting, and may not be able to make large modifications to exisiting fencing.
For digging dogs: Bury chicken wire at the base of your fence (with the sharp edges rolled inward), place large rocks at the base or lay chain-link fencing on the ground.