This is possibly the hardest thing Forum Administrators' deal with in their career as Administrators', and is without a doubt the hardest thing to do on your forum.

To keep members interested in your site and keep them coming back, you need unique things. Just look at what we have around here. What other forums have a Knowledge Base which is added to every single day with several high quality articles? What other forums provide you with graphics from some of the best free graphic designers known on the internet? What other forums allow you to promote your site in such a way that it's impossible for you not to gain exposure?

That's what you need to do on your site. You need a Unique Selling Point. A reason why members should join your site ahead of another bigger, more active one. Reasons.

Also, you need to change things a lot. Add competitions, give away freebies, do anything that is regarded as change and you will succeed. Changes happen a lot in real life and should happen in forum life too, changes are what keep people interested, would you go back to the same old plain site every day of your life to see no changes happening whatsoever?

The last thing to think about is, would you be active on your site? When deciding this, don't give yourself an advantage because it is your site. Think about other forums. Imagine, if your forum didn't belong to you and belonged to somebody else, would you continue to go back there and post? If not, then you need to look at what you would change in order to be more interested.

Think like a member, not an Administrator.