Signs and plenty of them. Nothing worse than not knowing the way to the next tee or if the tee box is not clearly marked, that you are on the correct tee.
Nope, can't say I agree with lots of signs, they make place look cluttered and ugly. The green keepers wouldn't be to pleased having to lift them all before mowing either.
One thing arising from the lost ball and playing out of turn threads is people who walk 50 yards across or up a fairway to help someone look for a ball but don't play their own ball first. Such a waste of time watching them walk back to their own ball after the other ball has been found.
I always say, "I'll just play mine up, then come across to look" if it's appropriate to do that.