we put in a credit card machine, any new clients get NO credit, and it also helps when your looking to collect money, as the client mite say he's to get to the bank or post cheque and then just say you can save them the bother and that you take credit card....
Also, i had a client last week, that was spending 1000euro with me, and said he wanted credit, i was afraid to give him credit, so i asked him for money upfront and said, i can give him discount if he pays up front, and gave him the job for 950euro, most client that are going to pay will jump at a discount on paying up front