Several things:
I would increase his prices, to cover the risk.
It also depends on what the layout costs to you are. YES you have to get paid but sometimes a $2000 job requires only an outlay of $250. The rest is time and profit. YES it's nice to get the $$$ in to pay bills, but the guy is actually paying, albeit "late". How long can you carry $250? If he doesn't pay the outlay to you is $250 but you get to write off $2000 as "Bad and Doubtful Debts".
Someone else could order a job on Monday, give you a chq on Tuesday and have it bounce on Wednesday.
All things being equal - sit him down, tell him that you have been "burned" and come 1st January 2011 all orders need 30% down, or full payment within 30 days to which a discount applies. That way he has a choice to either use you or not. If he knows the rules he gets the choice to play the game or not. If you are a good supplier - chances are he will stick with you.
If he does choose to use you and doesn't pay (within 30 days) the first time after 1st Jan - then it's 30% up front. No exceptions.
Good luck - gut feeling is everything.
Cheers - G