The principles behind a successful referral program are similar to reseller programs. The key again is in keeping with the program . Contests work well with users. People are competitive by nature. Offer a large prize for the subscriber who brings in the most new customers in a given time period.
Free months are another inducement, but cash is more tangible and tends to elicit a better response.
Affinity This marketing technique takes a bit more care. Start by working with nonprofit organizations and computer groups. Typically, such group will not want the revenue from the users you sign up, but they do like to be able to pass on some savings . Seminars work well with affinity groups, and you can usually present your service to a 'captive' audience at their meetings. You also want to be in their newsletter if they have one.