Watch your ball until it stops or you can't see it any longer. I usually play with my three best friends, and one of them never (and I mean never) watches his ball the whole time. He can be in the middle of the fairway and ask where his ball is. And sometimes, when we are not in the same cart, he will come all the way over to where I am, whether I'm getting ready to hit or not, and ask if I saw his ball.
Another rule, one of my friends (only plays with us sometimes) will search for his ball forever. I swear, sometimes I think he takes more pride in finding golf balls then shooting a good score. He will slice a ball deep in the woods, and you better believe he is going to find it. I can understand looking for your ball for a litle while, but this guy takes it to the extreme. And of course, he always comes out with more than his ball. This is a quote we usually here from him everytime we play: "Man at $.50 a ball, I just payed for my round in golf balls I found." We will have to make a time limit rule for him