Start early. "Make it a habit before he's old enough to challenge you," says Laura Jana, M.D., associate director of Boys Town Institute for Child Health Improvement, in Omaha. When he's still a baby, wash his hands at the sink when possible after changing his diaper so he eventually learns to connect pooping with hand washing.
Give him a step up. Keep a sturdy step stool in the bathroom so he can reach the faucet. You won't have to lift him, and he'll relish the independence.
Make it fun. He'll be more willing to get sudsy if the soap comes out of, say, an Elmo-shaped dispenser that's easy for him to use.
Take it to go. Keep alcohol-based cleansing gel or wipes in your bag for when you go out.
Don't overdo it. There's no need to sweat every germ. Just make sure he washes up before eating, after using the potty, and more often if someone in the house is sick.