I've been planning to do this for quite some time. My electric fence around my tiny garden has long suffered from posts and insulators that were too flexible and stretching wire. It looks sloppy, and the bottom wire touches the ground -- not excactly what you want on an electric fence.
Earlier this summer I replaced the posts with more sturdy ones, and the plastic insulators with ceramic ones.
But the stretching still happened. So a few-years-old plan has now been implemented.
Each stretch of wire now has a spring in the middle of it. I got some pipe bending springs from Hoods for 88 cents apiece, and cut them in half because ... hey, the spring doesn't have to be THAT long, and half a spring is still a spring. Then I bent out the end coils on each one to form hooks, snipped the electric fence wires in the middle and made loops to hook the springs on.
Voila! Tight fence.