I agree with Lynn, It can be a very time consuming task.
I started a forum a little over 12 months ago now in the very competitive Internet Marketing field (Just like Lynn's) and knew that a strong effort would be needed to ensure that we survived.
The biggest challenge these days is maintenance of the forum, to ensure you keep the quality up you have to be ever vigilant and delete inappropriate posts and deal with those individuals that sign up to the forum simply to get their link out into the market.
These are mostly from those that run adult related or medical sites, I have deleted over 40 signups already this month.
The forum has grown and is frequented by some well known names from time to time however my ultimate aim was to ensure that the quality of posts remained high.
To me membership numbers are not important, I gain more from the sharing of information and learning from others.
If you practise patience your forum will grow and become more active.
Good Luck.
Cheers
Kevin Robb