What is a normal payment term?

Up until now I've always set my default payment terms to 30 days but I'm finding clients take that quite literally and send their payment 9 times out of 10 by the 30th day. I'm thinking, to expedite payment and keep a more steady cash flow going, to shorten my payment terms to 15 days. I am a freelance copywriter. What does everyone else who is a service provider set as your payment term? Is 15 days too short? TIA

via Small-Business-Forum.net http://www.small-business-forum.net/accounting-taxes/10586-what-normal-payment-term.html