Getting people to post in your forum is probably the hardest thing to do. First, not all of your readers will post messages. In fact, only a small percentage will post regularly, and most will never post at all. Second, nobody want to be the first person to post a message. In fact, topic starters (people that start new topic of discussion) will be a small percentage of your posters; the rest will simply reply to topics that have been started.
In order to get the forum to a point where it is self-sustaining, you will have to do a lot of work. Here are some tips that you can use to get your forum rolling:
If nobody will start a post, start it yourself
Register yourself as several members (you with multiple identities) and talk to yourself. (Note that this will not work in forum with logged IP). This will create enough "seed" posts to get people to read and perhaps add replies of their own.
Find topics that interest people
If you get a mental block and cannot find anything interesting to talk to yourself about, then go to other web sites and see what people in those communities are talking about in their forums. You can also look at usenet newsgroups. Digest these ideas and regurgitate them using your various identities.
Advertise your forums relentlessly
Make sure your forum is accessible from every page of your site, particularly the index page. Make the link highly visible, or outright talk about it on the home page. Add a link to your web site and forum as part of your email, usenet and forum signature. Frequent other forums and mention your site and forum from time to time (be careful to comply to their rules and not to spam).
Use your forums to support your web site
You can ask your visitors "What do you think about this article?" (or tool, tutorial, etc.) and ask them to post their comments in the forums. Additionally, use it as part of your customer support program. For instance, I use our web site forums to support our free PHP scripts.
Use your forums to annouce new changes to your site
This is a great way to make announcement because people can write their thoughts about the new feature (or article) you've just added.
There are many webmasters that have the exact same delimas as yours. Look for sites with empty or low traffic forums and email these webmasters. Offers to join their forums and post a few messages, then ask them if they can return you the same favor. If they want to know why you are bothering them, ask them to read this article.
Even if they don't have a forum, offer to write about their web site or add a link to their site in exchange for some posts.
As your member population grows, find a few good posters (the ones that post high-quality messages and replies) and ask them if they want to help you moderate the forum. Give them more rights than other users so that they can edit and/or delete other people posts at their descretion. They will help you keep the forum in order, make sure all the rules are followed and help you answers questions. Many people are happy to take on this challenge.
Do email follow up with your members
Send email to members that registered, but have not returned in a long time. Tell them what has change since their last visit and why it may be worthwhile for them to return to your site. Try to do this about once every quarter, but do not do this more than once a month. Be sure to respect their rights by including appropriate unsubscribe information and take their name off the mailing list at their request.
Some forums like this one offers incentives for forum members to make posts. For instance, we offers free advertising in exchange for posts.
Hire a Professional
This would be my last option, but it may work for you. There are professional message posters out there that will participate in your forums by starting threads and posting replies. This is a good solution if you are really stuck and need someone else to help you get the forums going.
Some other things to keep in mind is that you should start your forums small with perhaps 3-4 categories, then slowly expand and create more categories. This way, the forums does not look so empty. Additionally, some forums software has polling features. Use these to your advantage. Controversial polls and topics can create flurries of activity.