I have a friend that is the city manager of a town of about 25,000 where his main task is processing requests for building permits. Actually a volunteer-type job, no salary. But he makes a bunch of contacts every day, and his address book is huge. So he is busy all year except the Holidays. To fill in this time he started a Christmas tree lighting service (houses, lawn ornaments, etc.). In this 3-month period he makes enough to keep him going the rest of the year.
One of the most important internet marketing strategies is to develop every customer's lifetime value. At least 36 percent of people who have purchased from you once will buy from you again if you follow up with them. Closing that first sale is by far the most difficult part -- not to mention the most expensive. So use back-end selling and upselling to get them to buy again: