When our clients didn't pay us, we stopped all work on that project until we had their money in our bank account. That's really the best incentive: send out invoices once a month, and stop work on whoever doesn't pay you. People don't want to see all that money they paid go down the toilet. Getting 10-20% up front eradicates situations where people who don't want to send you the last payment for a finished project.

There's also nothing wrong with actually paying them a visit and politely demanding your money (only if they're in town of course). This can make you seem a bit more like you actually exist and you're not some piece of paper asking for money.