Once your site is up and running and technically optimized you'll need to build in by encouraging people to register and post. There are a variety of ways you can do this, and I'm going to list them all.
The first thing you should do it is make it easier to register, or at least explain the process a little better. Believe it or not but everyone isn't familiar with forum software yet, and if someone picks your forum as the first one they want to try to use then the registration experience can be daunting for them. You can alleviate this by giving them more detailed messages and feedback.
This contribution at vbulletin.org does just that. It displays on the top of the forum varying messages depending on who a user is. If it is an unregistered user it displays enouragement to register. If it is an unconfirmed user it explains the confirmation process and provides a link to resend the email confirmation. If it is a user who has not posted yet it displays advice on how and where to make your first post.
When I installed this contribution my registration rates almost doubled and I greatly increased my forum activity levels as well. It was also a painless install, taking maybe 10 minutes total. I personally think this type of functionality should be done by default, but until it is, this contribution is a must have.
Making Your Forum Seem ActiveThe most common way for someone to participate for the first time with a forum is if there is an existing thread or discussion that they wish to participate in. The second most common way is when they need to ask a question and feel that they will get an answer at your forum. If you do not have any (or many) existing discussions and if your forum does not look like it has knowledgable and active members then neither of these scenarios will come to pass and you will have a hard time building your forum.
Really, forums need a critical mass of posts and members before they really start to grow, and getting that critical mass can be quite hard. There are however shortcuts you can take. Now the purpose here is not to be deceptive, but rather get over that initial hump at the early stage of a forum's growth.
In addition to simply asking your friends to come and post on your new forum, one method is to ask your imaginary friends to come and post on your new forum. Meaning you talk to yourself using different usernames. It sounds silly but it works. The more you do this the better, though it can become boring and time consuming. If you're up for it though you could do as many as one thousand posts this way.
Another thing people have done is to import usenet archives into their forum. There are a few contributions at vbulletin.org that can handle this. Basically you take the public domain usernet archives and import them as threads and posts. I've seen people create forums as large as one million posts doing this. Obviously this method is a little shady since you're just making duplicate content. Search engines may even consider it spam. However I think that in very limited quantity (perhaps just a few hundred posts) this method is worth considering.
Paying for PostsLately, as in the past 18 months or so, a new phenomenon of paying for posts has developed. Basically in order to get an initial critical mass of posts, or in order to just increase forum activity, forum administrators have started offering cash incentives to their members.
There are of course prizes and contests like what you'll find at any business looking to increase their customer base, but also some ingenious ideas like Adsense revenue sharing. Google's ubiquitous Adsense program makes a perfect vehicle for giving back to your community. It works in general by allowing users to input their Adsense ID in their forum profile and then you can configure it to use that ID a portion of the time in threads where the user participates, basically sharing with them some of the advertising revenue, only you don't need to worry about paying them or doing any calculations, its all automatic and Google cuts the checks.
Now some people worry that Google does not allow this, but as far as I know, they do. People have emailed them asking permissions explicitly and they have said yes, they've also never to my knowledge requested a forum administrator stop doing this, and some well known forums do this.
If you don't want to share advertising revenue you can just pay for posts directly. Many webmaster forums are populated by people willing to post for cash, you simply need to make a request. Of course then you have to deal with hiring each individual poster and personally that sounds like a major headache to me. I prefer using a forum posting service.
With the way things have progressed in recent times it was inevitable that people start forum posting companies. These companies handle all the dealings with the writers, you just pay a one time fee and they do the rest. I've previously reviewed two such companies here.