Basically, keep things interesting, and keep in mind what users want. If your forum is growing, has activity, and content, keep doing what you're doing, otherwise, it will take something else to bring it down.
There are many factors that need to happen in the right order to build up a forum, but usually it only takes a small number of factors to leave the forum in ruins. From my personal experience, one or more of these things will be a major player in killing a community:
- Bad Staff
- No new content
- No way to get more members
Bad staff is an issue I have first hand experience with. A great forum with terrible staff is a terrible place to be. Quite a while ago, I quit a volunteer technical administrator position because the site was, simply, going nowhere. Constant talk of improving was as cheap as it was plentiful, but nothing got done in the way of actually maturing the staff. Needless to say, the place is in ruins now, with staff who are unable to make accurate judgments based on situations. If you are the community manager and you can see potentially bad staff, improve them while you have a chance, or be rid of them. It seems cold hearted, but this is business here. Don't let your forum fall victim to such things, it will destroy the members' faith in the leaders.
Content is key to any website. If your forum is your main feature, then the content is mainly based on your forums and how your users use them. This also has a downside: your members don't post, your new content ends. If they stop posting, the forum will die. Most medium to large sites have content separate from their forums, which makes them grow. Google likes this content, and it is the best SEO you can ever do to your website as a whole.
You can have all the activity in the world, but if only a few members are creating it, this activity can die very quickly. If there are no new members registering, then typically there are no new people generating content, and the ones that do can grow bored. This eventually leads to a group of people who lurk, after that, no new content is being created, and activity fades off into the distance.
Generally, if you can fill the niche your forum/website is targeted to and continue provide content for your users to absorb, your forum will have a harder time dying as long as there is something to discus. People will always have something to say, keep giving them that opportunity and they will grow bored as a whole less easily.