Previously, I had a 5 second nag screen at the start of every execution of the program and another 5 seconds at the end. On reflection today, neither of these sounded like they would ever be of use to me. I have a much, much better sales mechanism built in: every time the user tries to do one of my forbidden actions they get the error message and a gentle prompt that takes them straight to my purchasing screen. With that in place, why annoy people so much that they never get to the main screen?
I also put an easy-breezy button on my help menu to visit the website… which handily also lets me know via which URL they come by whether they’re registered or not. That way I can see if anybody is actually using my software, which is hard to tell now unless they’re one of the two people who routinely click the “purchase this now” button and then do nothing.