For me it also depends on the "reason" for the forum. For example, this forum is for "web mastering".
I ran a Counter-Strike Source (gaming) forum for 18 months. By the time we closed it down (and not because of inactivity) there was 700 odd members (30% active) and over 110,000 posts.
Someone suggested turning over fresh members to give the forum a burst of life... whilst yes, from my own experience a bit of life came from new members, it was mainly down the the regular posters interacting with new members that the forum would "come to life" on certain days.
As part of a gaming "clan" we were on other forums/sites for competitions, and as we always came across as friendly, others would visit our forum and ended up becoming regulars becaus of the amount of posts and the freedom of posts... What i mean by "freedom" is that there were sections on the forum where anything goes. A general chat section, an "gallery" (images) and a "movies" section - these became the hub of activity. Lots of general chit chat. I've been a very regular member of a few forums, and the one thing they have in common is definitely a general chat and images part to the forum. It starts general interaction where people can just talk about anything they want, or post up funny pictures. It also enables users to get to know each other better, thus creating a community feel.
I'm not a member of this forum for very long, but from reading posts on my first browse, there was a definite sense of community I felt - some users here must know each other pretty well at this stage, and at a guess I would say that's because of the general banter that's happened as opposed to "accurate" and "informative" posts. To create a community and more posts, there has to be a sense of informality some where (from my own experience).