Having a successful forum takes a lot of time and patience. I have come across forums where the owners feel like if they set it up, post a few times and then leave it, it will become successful over night. Unfortunately that is not the case and it takes a lot more work than that.
I feel that to have a successful forum you first need to have an idea and passion, you need to want to see your forum be successful. Once you have an idea and you have the passion it's time to move onto ensuring you have new content daily, interacting with your members as well is always good as they feel welcome and like they belong. With the fresh new content and the activity it's time to advertise whether it be on top sites, forum directories or through links in signatures on other forums it's all exposure and will help to get your forum out there.