TO CRACK, PLACE AN X IN THE BRACKETS [ ] (Score 1)

A simple nag screen on application startup should be sufficient.
Do not reduce any functionality from the application; there is no point, it will only drive the users away.

The nag screen can be legally removed by entering a simple combination of username / checksum (bob / 2013-028819)
Don't do anything cute like checking for time elapsed since application install, simply have it from the start.

Do not give the disassemblers any satisfaction by trying to implement a sophisticated protection mechanism.
Compile a string named "TO CRACK, PLACE AN X IN THE BRACKETS [ ]" and then check the string for [X] to skip the nag screen.
Then implement a crack by doing a simple search replace across the binary so that you do not screw up future releases.

Upload the crack for your own program to the PirateBay and place a comment in the crack stating that
you are the owner of the program and that you understand that not everyone can afford to buy your program,
but that perhaps they would reconsider once they use it for a while and start loving it. Be short and sweet.