Well, here are some of my thoughts.
I tried building a traditional forum-type community before. I used all of the right software and spent a ton of time making it look great and customizing form and function. One problem still remained: How to get it started?
At first I resorted to creating a bunch of users and, effectively, having conversations with myself. I think there is no avoiding that unless you happen to have a large group of trusted and subject-matter-knowledgeable friends and family who can help. The trust and subject-matter knowledge part are critical. It is easy to make a mess out of a community with dumb posts. And, of course, if you are creating a forum about chemical engineering it probably will not help to have your friend the plumber try to help you seed the community with posts.
So, in the case of the community I tried to build I spent a ton of time talking to myself as different users. I loaded-it up with dozens and dozens of posts. I also created other types of content that was easier to ingest and produce, such as topic-relevant quotes and the like.
Generating traffic and eyeballs visiting the site was still an ellusive task. I resorted to spending some money in the form of a small AdWords campaign. I think I spent $10 a day for 30 days. This succeeded in getting more eyeballs to the site. However, I learned very quickly the value of that saying: "You can lead a horse to water but you can't make it drink". How true it is.
Conversions to registered users were very slow. I think that a sustained campaign lasting perhaps a whole year might have delivered results as the size and variety of the membership would have expanded. The problem is one of participation. If you've been involved in forums for any length of time you'll know that only a small percentage of members actually participate and post. Most just visit and read. And so, you have a situation where you can actually gain a lot of members and visitors but precious few thread starters. The other side of that are people who are interested enough to spend a little time posting replies.
And that's really the key to the problem of starting a community: Gaining enough members so that the small percentage of members who are willing to post and reply becomes a reasonable size. Here the "90-9-1" rule rings very true to me:
http://en.wikipedia.org/wiki/1%25_rule_(Internet_culture)
And, so, I don't really have a magic bullet for this. I think one has to understand that --if you are on your own-- you will spend a lot of time creating content. You will be that 1% for a long time. As someone already mentioned, outsourcing content creation can be a very bad idea. If you are going to go with something like Mechanical Turk or the like to create content you will probably end-up with a very shallow pool of content that will not interest the 90% who just want to browse.
