Sure, you can employ a simple scheme where you write stuff in another part of the OS (like registry in Windows) and then rely on goodwill of your potential clients they will register your demo. But such a scheme is easy to abuse. So how about one that's a bit harder like having a self-modifying demo that will write stuff onto itself and check themselves to stop running after 30 days. Bonus points if your demo also has some database that is hosted inside the executable so it has legit reasons for doing the self-modifying bit. Even with a debugger attached you made to eventual cracker life 100 times harder. Sure, anything can be cracked but it's all about your hours vs cracker hours in the end.