I think a lot has to do with targeted audience and the market. If there is a similar forum like yours already have thousands of users then the chance is slim
I think a great way is to start promoting right here on FP. I just joined and I am already hooked. Its a great way to get new members - check out the exchange forum.
You might also have a contest on your forum. What kind of contest you should have would most likely depend on what type of forum you have. A contest might make things look more fun and exciting, while getting the members you already have to post a little more around the forum. Once the contest is started, you can advertise it whenever you talk about your forum.
I agree, if you create a contest on your forum, it would help a lot, you can promote your forum in other related forums as well, social media sites as well, these are the main method but don't forget content in your forum you have to create quality content regularly so that some visitors would come from search engines.