Most people can grasp the idea of the rear gears quite easily (perhaps because they last rode a 5-speed as a child) but are confused by the front. I've told people to stay on the middle ring for normal riding, using the whole range at the back, and treat the large and small rings as extensions of the range. Show them how cross-chaining is hard on the chain, and tell them which gears to avoid. The hardest thing is getting people to ride in a gear that isn't too high.