I got this trick from Yoda.

Tired of The Man sticking it to you with expensive cable modem bills? Then try this out:

Call your cable modem provider and ask to speak to someone about cancelling your service. After being directed to the appropriate department, tell the service rep that you received a flier in the mail for DSL service advertised for only $19.99 a month, and wanted to terminate your service to opt for the DSL. They will attempt to persuade you to stay by lowering your bill, but don’t fall for this trap. Tell him you want them to match DSL at $19.99, or it’s over. If they won’t comply with your reasonable demands, simply hang up and call back. You will undoubtedly receive a different rep this time, so give her the same pitch. It took me 3 attempts, but I’m now locked in for $19.99 for 12 months! via Yoda