You want to entice them to pay without causing them to be so annoyed that they stop using the product. Some people develop massively complex password schemes to prevent multiple users from using the same password key. You want to be careful about this. You don't want to make it so difficult for a legit paid user to move their software to their new hard disk, etc. If your protection is really secure your normal paid users will be unhappy with the product and not recommend it to friends your time with support questions.
Some people provide very short trial periods for their software.
Few people will install some software and then devote the next two days of their lives to a massive testing of your software. Most people will install it, launch it, and see what it does then not use it for another month until the need for your software presents itself to them. Do not use short trial periods. Instead consider actual hours used. Do not use a specific calendar date to determine when the software should cease to function.