If you are in a cut-throat business climate, why not build in some bugs that popup and disable the software on a regular schedule, requiring them to hire you to fix it. (Just like Microsoft :-) ) You could obfuscate your code enough to make sure that they would have to hire someone to de-obfuscate it, so they may as well pay you.

Another possibility is signing longer term agreements, with penalties for early cancellation.

If that fails, just put a higher price on your product, and tell them why.