a few
first make it an mde, so they can't see your code
then decide how you want to do it
basically
a) make users login, and limit the number who can log in OR
b) make users register the app, and tie this to something on their system (eg HDD serial no)
c) even just install a hidden file, or something that your app checks for.
might be some other general ideas
how you implement all this depends whether you want it just not to be too obvious what you've done, or whether you want to be as uncrackable as possible. There have been a number of threads about this on here with a lot of info already.