[Mendax]

-> Keep making contests

(This will want people to sign up!)

-> Advertise legally on other forums

(By legally I mean through webmaster showcase & review threads, displaying your site in your signature (if allowed) )

-> Update regularly!

-> Focus on the non-premium/premiums.

Even people who aren't VIP may feel left out, make them feel special and this will help them stay.

Thanks and have a great day.