Provide information worth signing up for. I run a large mead-making forum at Got Mead - Mead (honeywine) making, mead drinking, mead recipes. Most folk run there as non-posting guests or registered members, but I encourage paid membership as well by offering patron-only forum boards and recipes that have been proven.

I encourage registrations by not allowing guests to post, only read. Likewise, many areas of my main site are only available to registered members.

I get on the average 10 signups a day, and 15-20 new patrons per month by offering them custom content, chat and a patron-only magazine.

The secret is content, mainly. Lots of unique content.