The Wall Street Journal has a great piece today on how to get what you're owed by clients. As always, the best remedy is prevention: setting clear expectations about payment when you work out your contract. But if the person or company using your services doesn't abide by the planned payment schedule, you can do more than just send a strongly-worded email. Some tips from the article:

• Offer incentives: Maybe a client who pays early gets a small percentage off the bill.
• Offer penalties: If the contract specifies late fees, send a reminder of the consequences along with the first bill.
• Call and write: Don't just send a reminder by letter or email.
  You can also check out--and contribute to--our forum thread on clients who don't pay. When we surveyed independent workers about this issue in 2007, 77% of respondents reported experiencing a problem with nonpayment at some point, so there should be plenty of resources right here from our membership.