Also, I suggest that when its just starting out, instead of hoping for people to join, if you see anyone on sites that are interested in a topic, you should ask them if they would be interested in joining (just don't make it like spam advertising).
Depending how big/popular or how much 'power' your forum has you could also strike up deals with other websites and companies.
I am currently in talks with a reasonably new but very popular media network which have various fan sites for things like Big brother, X factor, Britains got talent, general TV chat and such - yet they have no forums. So it may be a good idea to see if you can find websites about your topic and offer them your forum to use.
It's very important you have a niche - a unique selling point if you will. It's OK to have a topic or theme similar to other forums but it has to have something that differentiates itself from the rest - something the user will get from your forum but not from competitors.
You should put yourself in the shoes of the visitor - If you were him/her, would you want to join? If there are aspects you reckon you would disagree with as a user than improve them. Good luck.