I\'m working on an app I\'d like to sell some day -- sooner rather than later! I\'d like to develop a reasonably simple serial number scheme to protect it.
All rules whether custom developed or encryption based can be found out and cracked. Think about who you're building your application for. Many of my products are for businesses so they're either going to buy it or they won't. They're typically not going to run a hacked version on their networks. People looking for a keygen aren't likely to purchase your product regardless. You just want to make sure you don't annoy your customer to the point where they're no longer wanting to buy your app.
That being said, I've written a library for this sort of thing to use in my applications based on AES encryption. I'm selling it for $25, and it uses a passphrase and a salt to make your serial number unique. If you're interested you can find it here: http://simpleserials.com