You need something unique, you should figure out some answers to the following questions and answer them as a method of promotion and maybe post it on the "What's going on?" forum. Unfortunately people wont 'just join', they want something in return. Maybe rewards, awards, forum currencies, etc.
Why are you unique?
What will I get there that I wouldn't get on other promotion forums?
Answer those and you're on the route to success! (hopefully)
Good Information
Some things that can help too
1) Consider a redesign
2) Create content – and lots of it
Your forum probably died or probably will die a slow death due to a lack of content.
3) Set yourself targets
4) Email current members
5) Continue to market, create content and nurture your community
Time is against you
Not everyone has a lot of time to spend in your community. Parents are busy raising their children. Fishermen are busy fishing. Gamers are busy playing games. They need a reason why they should give up their spare time to contribute to the community.
Advertisement and get the word around.
Get the active members active by doing giveaways and making them freedom to the website.
Ask them what they want more then what you want.
Keep content going.
Stay alive with your members and get talking with them.
Learn on your mistakes and grow with your website.
