Some chickens (especially the lighter breeds) can actually fly quite high. While this might not be a problem (I have a chicken who loves flying into trees, but stays in the backyard where it's safe), sometimes it requires some creative problem solving to make sure your flock stays out of harm's way.
If you're having a hard time keeping your flock in their designated space, one solution is to clip a wing. Just one wing, not both, of each chicken with a flying problem. This throws them a bit off balance (when it comes to flying high, not walking or other things) and will keep them from flying higher than about 3 feet off the ground. (They'll still be able to fly up onto the roost.)
This clipping won't hurt your hen. (It's like a haircut, not an amputation.) The feathers will all eventually grow back in. You may or may not have to re-clip the wing feathers depending on if the hen has adjusted to life without soaring over the fence, or whether she will think to attempt it again.
To clip your chicken's wings, you'll need a pair of scissors. You might need the help of a friend (one to hold the chicken, one to clip).
When clipping, make sure you're only cutting PRIMARY feathers. These are generally the first 8 or so big feathers on the front end of the chicken's wing. Spread the wing feathers out and carefully cut those 8 primary feathers off while leaving the secondary feathers intact. Remember, just one wing will do.
That's it. See, I told you it was easy! If you've got a wanderlust chicken, clipping her wings might be the best thing you could do for her. This will help keep her from wandering into potentially hazardous places and could save her life in the long run.