Interesting topic: Web Hosting: Ideas on Forum Activity

After choosing a niche, domain, hosting, and software, many forum owners think they don't have to do anything else to make their forum become popular. This is not the case. The popular phrase, “If you build it, they will come” does not apply to online communities. You must work to get your name out there and attract members. There are several popular methods of doing this.
The first method I will talk about is a very common and popular method: post exchanging. Basically, you agree with another forum owner to post x amount of posts on their forum and in return they will post the same amount on yours. This is in no way a long term solution but it makes your forum appear active and more appealing to guests who might have a genuine interest in joining. Its easy and helps both parties out. The problem some forum owners who use this method face is using it as their sole source of generating activity. You have to attract real visitors if you want your post exchanges to pay off. A similar method to post exchanges is paid posting. Personally I don't have any experience in paying for posts but its a popular service. You pay a certain amount of money and in return a writer or writers will join your forum and post a certain amount of times. I prefer paid posting because it doesn't take up a lot of my time and is free.
Another way to attract users is to submit your forum to directories. And when I say this I mean a lot of directories. You have to get many links out there on different sites for it to be worth it. There are many free directories out there specifically for forums and also specifically for your niche, and there are also many general directories. It is worth it to submit to as many as you can to get your link out there and get backlinks.
A great way to attract more users is to place a link to your forum in your signature at other, related forums. And don't just put the URL, put the forums name or topic as the anchor text. If you make quality, helpful posts people will generally click your link if they are interested in the topic. Forum signatures is probably the easiest and yet best ways to get users to your forum.
My personal favorite way to advertise is to use social media websites such as digg and del.icio.us. I submit many threads to digg and it generates a lot of traffic if your thread is interesting and unique. Remember that digg is mostly technical related though so not all forums can take advantage of it. Other sites like these include Netscape, reddit, and StumbleUpon.
The final way of generating activity in forums that I will discuss is having a contest. A big contest can bring tons of users to your forum. Some may join with the original intention of only joining to win the contest and then leaving, but if you provide unique and interesting content while they are actively posting you may change many of their minds. Contests are probably the best way to generate significant amounts of activity in your forums. If the prizes are good enough then they can guarantee many new posters to join your boards.

From http://jack-fx.com/tech-article/100.htm