Contests work well with users. People are competitive by nature. Offer a large prize for the user who brings in the most users in a given time period. Free months work, but cash is more tangible and tends to get better responses. Prizes tend to work well because it is a tangible item that the user would want.
Affinity programs take a little bit more care.
Start by working with non-profit groups, and miscellaneous computer groups. Usually, the group will not want the revenue from the users whom sign-up, but would rather pass the savings on. 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. In offering e-mail at the association's domain name, you will have a better response, and you have a retention tool.