The main thing is content! I have only about, what, 120-130 members maybe (too lazy to check for an exact number) at the moment, but I bet that every v-drive boat enthusiast who's used the internet has seen my site, and probably at least one-third of those people have registered! (I have a somewhat off-the-wall topic here, gimme a break.

) And it's all because of content.
Secondly, I noticed a huge jump in the registration rate when I redesigned the site. Visual appeal, I think, really does have a lot to do with it. Your site looks rather amateurish and commercial as far as graphics and colors go. Work on that a bit, and you'll probably see a lot more participation.