[obviouslygreen]

Use a contract and set your payment terms clearly; bill at defined regular intervals (with a partial up-front payment).

Make sure there's a late fee, and enforce it! You will almost certainly find clients that don't pay on time, and letting them essentially get away with it will only make it worse.

The one measure I've found that puts honest but lazy clients on the ball, though, is stopping work as soon as payment is overdue and not continuing until they're current (including the late fee).