We've done a couple of things - the golf balls have worked for us. Also, as time goes on they tend to wander a little more - they have basically a horse stall with "laying shelves" (a board about 16" wide x 8' long with a 1' back and a 2x4 front), and the door to the "coop" rarely gets closed. When they begin to lay enmasse outside, we just keep them in the coop for a couple of days (they do have a small screened run to go out into), that seems to "remind' them where to do their business.
Good Luck!