It's easy if the games are meant to be played online only... perform part of the game calculation only on the server, and use logins. But if you have a single-player mode...
A good "minimal" protection would be to use a hardware fingerprint and a matching HMAC downloaded from the server. When the user moves the game to a different computer, they have to use their credential to generate a new HMAC. The game itself doesn't contain the private key necessary for HMAC generation, only the public key necessary for verification. This will prevent accidental unauthorized copies, and there's very little you can do about skilled crackers anyway.
It's probably a good idea to include sunset logic, in case you shut down your server someday, you don't want your paying customers left with no way to reinstall. The likelihood you'll still be seeing significant income from sales more than 3 years after release is miniscule. You could also release an "update" with the license checks removed at some point in the future if you get tired of checking licenses.
For some extra information on why you don't want to join the arms race, see this answer of mine: The #1 law of software licensing
