The easiest way to launch a new forum is when it is launched on, or in conjunction with, a preexisting popular content site. This way you already have many regular visitors and it should be fairly easy to turn them into regular forum posters as well.
There are a few tricks to increase the conversion though, one of which is to feature forum threads on your homepage and elsewhere. This is probably the most common contribution or hack installed for forums, the one for vbulletin 3.5 can be found here.
In addition to your homepage, if you have a large site you can take this same concept and use it on category node pages or other pages that have a corresponding forum section associated with them. At the very least you should be providing tech links from sections on your content site to corresponding sections on your forum.
You can also use the forum software to build a user comments system for commenting on your content or products. The typical "user comments" seen at the end of an article. This will again encourage your existing visitors to register on your forums. Although you will receive less such comments than if everyone, not just forum members, could post.
Further Reading & Resources: