User licensing/registration schemes?

Hi Folks,

I'm considering selling a plug-in for about US$50 to US$100 (not too expensive, I think), and would like to hear from you about how you would keep users honest when they install it. In other words, how do you protect yourself from a user purchasing a one user copy and installing it in his/her shop for multiple users?

I imagine that the more expensive the plug-in is, the more sophisticated one can get with protection schemes without annoying the user too much...

Thanks for any suggestions.

-- Jim