Incentive, thats pretty much the only way.
Write in the contract that if they pay within 2 weeks of the actual shoot date, then you give them a 5% convenience discount.
Most companies know they have to pay anyway, so they'll pay quick and get there $71.50 (average job cost) in discounts, depending on of course how much the job costs. Even a small job, you'll give them say a $12 dollar break on it. And I mean whats twelve dollars you think to yourself, well what's two minutes to have your accounts people draft up the check today instead of tomorrow?
Some clients will be to busy or won't have there act together, and will take longer to pay. And those guys miss out on the discount, fare and square.
Generally speaking, most people will pay you damn quick for the discount.
Oh and the people who don't pay you, but publish your work anyway, well stupid them, because they can get into heaps of trouble. Well again depending on what your contract states.