Gather the preferences of your customers and monitor their buying habits. This can provide you with very valuable information. To get valuable ideas, look at some of the things Amazon does. For example, Amazon allows people to subscribe to updates on various different types of books. That way, they are able to provide information on a regular basis that the customer is interested in.
Most programmers spend some of their spare time on personal software projects.
Unfortunately, most pet projects never earn the developer any serious money or recognition. Now there's a new alternative. Amazon also has an author update service. You tell them who your favourite authors are and, whenever there is anything new by that author, you receive an email notification. They also have a facility, where for the book you are considering purchasing, that lists suggested other titles based on what other people bought.