Offer something other forums do not. I do not know how much space you have on the server but otherwise you could host some rainbow tables for example to attract people.
If your referring to the theme, we have a custom theme being coded, we are also adding Hacking CHallenges, youtube tv episodes (hacking tuts ), and more, so if you have specific suggestions feel free to add them!
Hahaha, no I made this post before I actually read more about what you are planning on. I think you are going in the right direction unlike most of the people starting these kind of forums. :D
But then to be honest... most of them could not separate a string from a word. XD