How to advertise an online RPG and actually get members?

I help run a magic school online forum-based RPG. I’ve done all the advertising I can do, but we’d still like to grow. We also seem to have people who register, but then don’t return. We welcome them as best we can, but if they’re not around, we can’t get them to join in games or anything.

Any ideas of how to get people to join Salem Academy of Magic and actually be active?