I have a 3yo Black Lab/Australian Shepherd mix that liked to do the same thing.  When I bought my underground fence, I purchased a system that came with a remote trainer.  Although, I didn't really use it for training him (very very stubborn dog), it has come in handy when I hike or backpack or camp or want to have him off the leash.  Because he is already conditioned to the fence, this works really well. It has a pretty good range also (1/4 mile). Innotek makes some great systems. Check out Cabella's or Innotek.com for info.