Very right, and this can be the NB factor for now.
First you need a forum that has an interesting subject. No one wants a boring forum.
Time. Time. And more time.
Promote via word of mouth if your forum is for local.
It took mine over a year and half to get to where it is now. :D
And I realised my forum members prefer simple forum theme, instead of the more advanced and complicated design. So, I had mine changed a few months back.