though contest too
tech forum actually gets members usually if the posts are helpful and solve thier computer problems.
How do you get members? As I just made a long post over at TheHazey, I felt inclined to make this topic. Anyway this is what I think.
There is no definite way to get members, at least in the long term. Promotion forums can get you members but they can only take you so far. And chances are, you won't get many active members through promotion forums, only people from post exchanges and packages.
Also try and have a keyword friendly domain or a Web 2.0 domain. The first can bring good traffic from search engines and potential members if you do the required SEO. The second may not be searched as much on Google but a catchy, clever name can definitely increase the chance of getting members.
Another method is to have the link to your site in large administrator forums/webmaster forums if it is permitted and post around of those kind of sites regularly.
Also it may be worth trying http://paidtoreg.com, however you need $10 for posts and it will take a while to get that much. Once again though it is a short term method rather then a long term method.
You should also try and have competitions and offer incentives for members to post. I do this through the make 50 posts, get a free .info scheme which quite a few people have posted for and requested.
These are some basic pointers which may get you members, but really members do not mean much if they're not active.
Over at TheHazey we are offering a summer sizzler competition where the member who posts the most in a month wins some money. Great insentive to all our members.