Personally I hate copy protection and most DRM (Digital Rights Management), it never works or it's a pain if you plan to use it on multiple systems or you change hardware etc. Here are some tips you should consider:
1. Use some sort of reward system. (if a legit user then give them access to extra support, bonus features on your website, etc)
2. Don't be greedy, a single person should be able to run the program on as many of his own computers as he want.
3. Provide a multiple users or company license for cases where multiple individuals will be using the software at the same time.
4. Use a combined serial, both for user identification and feature enabler, re-using same serial for website login and features is advised. Clearly state that the serial is personal and that it is actually the serial that they "purchased".
5. Make sure that legit serials can not be automatically generated by third parties, use a server database to hold all "real" serials, this prevents fake serials from getting the extra features from the website/support. Never have more serials in your database than you have users, in other words, do not pre-generate serials or arithmetically pre-generate them.
6. Do not shutdown or block the program from running if a fake but functional serial is detected (server check), just record the fake serial and ip and let original functionality be available. Even if they use a fake functional serial (passes the program's internal serial check) they are still you customer (although a currently non-paying one) let them still use the basic features, as long as they keep using your software they are still future possibly paying customers, and they wont get attracted to a competing product and possibly spend money there instead of with you.
7. Be your own user, by this I mean, pretend that you bought your own software, do you find the DRM system a pain in the ass or nice? If you would hate such a system then be sure your customers will as well. If done properly the DRM should enhance the feature set of the software for legit users, and add nothing for illegitimate users.
8. Even non-paying users tend to brag or recommend software to other people, and those people may actually be willing paying customers, word of mouth and brand recognition is also worth something.