How are you getting traffic to forums?

I used to work full-time promoting/managing/building online communities in a former life.

The first thing I learned is that habits are difficult to change. If they're used to just showing up, jerking off, and leaving, it takes time to change that.

Newly beginning communities need to be "seeded" with discussions to help get things rolling. Just one or two starter "welcome" messages won't cut it. Most visitors also won't start a new thread in a new forum all their own at first. They'll comment on existing threads. If you can get the help, have friends, co-workers, or whatever help start things off to bring the forum to life. Have them reply to existing threads and start new ones.

People are less likely to participate in a forum that appears dead or empty. So seeding is also important for that.

Speaking of appearing dead, don't start off with too many sub-categories. The forum should expand organically. i.e. Have a generic "on topic" board and a generic "off topic" board. Being too large, too fast makes it look empty.

While seeding, make posts that help guide and continue the discussions along. Answer questions and ask new ones, or propose alternative ideas, etc. Whatever techniques you can come up with to get them to post again.

Don't be afraid of "hot topics". Something like politics will turn ugly pretty fast, and depending on the type of forum you're going for you might want to avoid that type of flame bait. But think of discussions that can evoke a reaction of some kind.

One of my favorite examples from the old job was creating a thread about how much I hated Jar Jar Binx, and suggested multiple creative ways of killing him within the Star Wars universe. It's a benign topic but still evoked an emotion. ;)