License registration is a common and widely used licensing scheme. It uses the Registration ID as a means to allow you to identify the application, as well as the user. Accordingly, it uses the License Key as a means to allow you to license the application.
In this method, when starting the protected application (or the Register utility), the registration dialog pops up showing a unique Registration ID. To register the license, the user must contact you (via phone, fax, email, website, etc.) and provide the Registration ID. With the received Registration ID, you can identify the application, and then generate the License Key to be returned to the user. The license registration is complete when the user enters the received License Key to the application. As a result, the application is licensed as the properties encoded in the License Key.
Since the Registration ID is randomly generated, you can ensure that each copy of the protected application will generate a unique Registration ID. Starting the application on a different computer will always give a new different Registration ID. Similarly, starting the application (on the same computer) after the license is expired will always give a new different Registration ID. Therefore, you can ensure that the user cannot reuse the same License Key to activate the license.
While the user must contact you to provide the Registration ID and to register the license, this method enables you to have full control over the distribution of your application. During registration, you may collect the user information as well as payment (through credit cards, PayPal, etc.) before issuing the License Key to be returned to the user.