I have two golf balls in each nest, and they are funny about them. One girl tucks them under her chin and rolls them out, and away. One girl pushes them off to the side, and the rest just sit right on them. No eggs yet though, but I know we are close! Good luck.
It wont make them lay any sooner but it will encourage them to lay in the nest box. I have one in each nest box and the girls use them consistantly. I have one barred rock who just likes to go in and sit on it for a while. Shes not broody, just likes to sit on it. Crazy hen.
I tried the golf ball trick and my chickens all caught on to laying in the nest boxes pretty quickly - just had a few eggs on the coop floor at first.
Some of my nests have a couple of golf balls, and I've noticed that the chickens seem to prefer laying in the nests that have more balls - i guess maybe they instinctively try to add to an existing "clutch"?