I had a car classifieds forum not too long ago for a particular make/model. I did a lot of things to drive traffic to the site. I know that you're in a completely different niche, but maybe you can get some ideas from what I did in the past.
1: Posted useful articles (what to look for when buying this car, the top 4-5 models from this make and why, which ones are holding value or becoming more of an investment, etc..)
2: Offered to post all of the dealers inventory on my site to get them started
3: Offered free "premium" accounts (free of ads) to people who were active or joined within a certain time frame (ie: first xx people.
4: Social media (twitter, facebook, instagram). Search for your niche and try and be helpful
It was a lot of work to get it going. I was averaging about 50-80 unique visitors a day but unfortunately I was using XenForo forums on a friends hosting account and he decided to close it down and it was just too big of a hassle for me to get it going again and getting everything transferred.
Hopefully this helps a little.