You have a forum and you would like to increase user participation and numbers of active discussions?
Here are 10 ways to increase it.
If your existing content is interesting, then people will come to see and read it... but only if they know it exists. Social Networks will do the promotion for you by spreading your interesting content to Twitter, Facebook, Digg,... So, think about these famous Facebook Like Button, Tweet Button, Digg it... that we see everywhere.
Replying to an existing topic is as easy as clicking on a Reply button... but the user's will could be immediately stopped if he never created an account on your website. He would then need to follow the subscription procedure, to type, again, for the fourth time this day, his login/password/email/age/accept disclaimer/go to his mailbox/validate his account... and finally forget what he had to say.
Why not instead asking him to connect through his existing account on his favorite provider: Google Accounts, Twitter, Facebook, Yahoo,...
Another alternative is to simply bypass this step by accepting anonymous posts, even if it requires more moderation and tends to increase the complexity to identify authors of posts. Indeed, a user could create dozen of messages before thinking about officially subscribe to your forum.
Avatar is the best visual signature of a user, and seeing his own avatar on the Top Poster or in the Most Popular Topics could be a great satisfaction for him... so display his avatar, as much as you can, and make it easy to upload to your website.
In addition to social logins, web tools like Gravatar could be a nice way to centralize in the same place the user's avatar. Try to support this kind of tools if possible, depending on the available mods and plugins you have on your forum software.
People won't come to your forum just for fun, and will not start a discussion about their private life or their future project just because the design is cool, and even less if the activity looks pretty low.
So, the best way to increase user participation, more particularly when you start your forum, is to create topics and launch discussions about subjects related to your forum. Of course, linkbait addicts would also suggest to post really specific viewpoint or minority opinions (like "BP is a great company", "Lady Gaga is an awesome artist" , that would increase your forum traffic with people that want to claim you are wrong...
Again, the drawback is that it can increase anger and moderation problems.
Many forum members like having new responsibilities, and the best one and the most distinctive is, obviously, to moderate a part of your forum. Do not hesitate to share your forum governance with others to have a more democratic community.
In case you don't want such, and prefer keeping an autocratic policy, tools like Post Rating and User's Recommendation are really nice to distinguish high class posters from spammers or read-only members.
Having a too generic forum, or talking about general topics, is not always worthy for shy people, that would not dare creating new topics fearing to be bashed by the community (okay, in this case, your community is more like 4Chan than Barack Obama's Official Forum). The solution is to simply split your forum in smaller categories, to really identify the topics of your forum. With this clear separation, people will feel more comfortable, as they will easily meet and recognize the regular users, and the discussion will be facilitated.
This one is more generic, but don't count on users to be patient. If a page is too slow, it has more chance to be clost, and you can say bye bye to interactivity between users. Do your best to have the fastest forum, even if it means disabling some features, like a calendar, birthdays list and so on. Additionally, there are several guidelines to increase the general speed of your website, at Yahoo Developer Website and at Google Speed Tool.
Do not hesitate to even propose useful tools to automatically post new images using image hosting services like ImageShack, because there is nothing more painful than uploading an image somewhere first before being able to display it on a new forum topic.
Sometimes, there is nothing better than a direct and informal discussion with your users, so, instead of waiting for new topics or replies, you could also think about discussing with the current connected members.
It will always be more simple and less formal, and it could be a nice way to discover why people do not create new topics on your website (they encountered a bug? the content is useless?)