Unless I know the client well, I get a 20 hour retainer up front (which is about as much as I usually bill any single client in a week unless I really like the project.) I send them a bill every week, and while I'll give them a little credit on getting it paid, I won't go more than a week without getting the retainer paid up unless we've got some history.

Sometimes you have to be firm on it, but trust me, beats the hell out of trying to collect 10-15k in outstanding invoices when your mortgage is due.