Many ISPs cite word of mouth as their largest source of new users. Referral programs are designed to help cultivate and grow the amount of word of mouth sales an ISP has. 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 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.