Hi Matt

Have it "call home" with a user ID and password you assign to valid users. Manage the passwords within a central database. That way if it gets shared by bluefarters you can simply disable the offending account.

I've done this on some of my designs and it works very well. Hope it helps with yours, and best of luck with your plugin.

Happy New Year!

Bill

Protection from hackers = already answered.
Protection from lawyers?
What about the lawyers...

FYI:
The Wordpress founder is increasingly radical about being "pure GPL" for all legit themes and plugins.

The future of paid plugins lies in projects with the Lgpl, MIT, and other similar licenses.
Good luck, I'm just trying to help.

There is a nice point that you bring there Happy_Balance. GPL does allow you to sell your plugins, but at the same time also allows your customers to redistribute it under GPL (possibly for free). However, if you provide good support, then people will buy from you to get that support, rather than using a random WP plugin.

Also, if your plugin requires an API hosted on your servers, then you can charge for the API. Even though people may give away your plugin, but everyone will still need a valid API license.

Regards,
Revolves