One of the hardest tasks forum owners/administrators face is converting their visitors into returning guests, members and, hopefully, contributors. Search engines and advertisements will bring people in, but keeping them interested and preventing them from wandering into the aether is another matter altogether. If, as I do, you run a forum that is supporting existing content, you've got an edge up to begin with: you have a good reason (presumably) for people to come to your site. The next jump to make is using that content to drive people into your forums.
My core site is a download hub for a musician with a large amount of live content that's legally tradable. I'm doing a heavy amount of traffic in downloads (though way less than I would be if I wasn't able to use the Internet Archive for lots of my storage - it's an handy resource for you site owners with media that you're already giving away for free), but getting those people in to post on the forums is a task indeed. If, like me, you have a forum supplementing a main site, it is essential to take full advantage of your existing content, and use it to best exploit it to bring new members to your forum. This is a lesson that I learned quickly, as I observed just how much traffic my site was doing, and how little of it was occurring in the forum itself.
When the board was first installed, I didn't even think have links to it from the actual content pages - just one link from the home page. I realized my mistake after a couple of days, and I added a "Message Board" link on basically every page. Really, you should always give your users a way to get to the forum, and if you have a navigation bar or map, at the bare minimum, you should include a clearly visible link to the forum index.
On this page, the Message Board link goes to my forum home page, the Reviews goes to a thread for people to discuss that recording, and I'm in the process of linking each song title to an appropriate lyrics thread in the forum.
Of course, if they don't have anything to say, this link isn't going to do you much good. You'll also have to give them a reason to visit the forum, at least to read, and hopefully to post.
One excellent way to encourage participation, or at least to get people into the forum, is to offer to opportunity for users to read and write reviews of the content, services or goods offered on your site. In my case, I added a reviews board for people to share their opinions (and read the opinions of other users) for the live shows in the downloads section, and followed up by placing a Reviews link on every download page. As contributions to this section come in from the posters, I will have valuable, unique content that users will visit my forum to get.
That's a user review there, and it's absolutely great. It's a nice chunk of content, and it's unique to my site, so there are no duplicate content penalties. It's because of reviews like that that I've been able to partner with another Elliott Smith fansite; the owner has a list of almost 200 shows for people to trade, and every one of those pages has a link to it's review thread in my forum
Odds are, you don't run the same sort of site I do, and you probably have a different sort of content than I do, but there's no reason to let that stop you. Pretty much any type of content you can post has the potential to be reviewed. How good are the products you are selling? How helpful was the advice or tutorial you posted? Do your posters like or dislike your latest song or album? Be creative; these are only a few examples. The principle is sound, and is something that can be implemented for nearly any site with content outside their forum.
Similar to a reviews forum, another feature that can be nearly universally implementable is a good support forum. Obviously, this is going to be more useful for those of you who are offering certain types of products and services. Beware - these users will be some of your most challenging to deal with initially. Provide them with the support that they need and (and a good community environment) and some of them will also become your best contributors, when they turn around to share their knowledge with those who come for support on that issue next time around.
If you are just adding a forum to your existing site (or working to improve traffic for a poorly performing one), consider shifting some of your content into the forums itself. For starters, this will bring more of the visitors from search engines right into the forum itself. Allowing your users to interact with it gives it a chance to grow and increase in value. For example, a plain list of tour dates on your bands website is certainly useful. Now picture a list of tour dates, linked to threads in your forum allowing detailed additional information (directions to the venue, costs, age requirement, etc) and a place for users to organize in advance and chat about the show afterwards.
That's a Dave Matthews fansite there, and they're doing an excellent job of implementing those sorts of things. Sadly, since Elliott is dead, I can't use that idea
Again, this is just one example; use your imagination! I obtained permission to post lyrical content, and I've created a discussion forum around it, added the content itself to the forum to bring users from search engines directly to my forum. I also link any individual lyric which is under debate or the subject of a thread in the discussion forum to the discussion thread talking about it. My next step will be adding links to each song lyric from every show page on the site.
Really, all of this boils down to a main point, and a couple simple corollaries. Above all, remember to make your content work for you. The longer you keep a visitor exploring your site, the more likely they are to register as a member. Whenever possible, you want give your users an internal link that they can follow that will keep them browsing and do your best to give them a reason to post. After all, in the end, unique content is king on the internet and nobody can do it better than your forum members.