I think any forum has to have a variety of topics and activities to cater for a wide range of people. e.g. hobbies, games, world news, politics anything really. A lot of forums seem to have topics as e.g. "What did you do today" Not an in depth topics where you can type an opinion.
Agree the hardest part is getting people to come back to your forum
Lot's of advertising, but don't take it to far, you don't want to look desperate or like a spammer. Also, NEVER LIE, maybe don't share all the negative aspects of your forum, but don't lie.
But the biggest one, is patience. And waiting, lot's of it. It will take time for you to get the active forum you desire.