Where are you trying to get traffic from? If you are trying to get traffic from other websites you may have to change your marketing plan. The only way to get maximum long term traffic is by ranking high in search engines for keywords that get searched often. Basically, build strong quality backlinks, write unique and helpful content, and you can rank for your keywords. Stop investing money, you don't need to invest 1 cent in marketing in order to get a number 1 ranking for a good keyword phrase. It just takes some work and time.
Observe people demand in what they interested and then make your forum section according to that. In your forum section put topic related to your field and also people interest topic. Start get people for your site and then discuss with them in their point. And for promoting your site you can go for blog , articles , videos and many techniques.