IMHO, the best way to attract people to the forums is improving the site with fresh/relevant/technical information, as David and some others already suggested.
I am trying to work on many fronts in maintaining/improving the backend software/server, adding some news, articles, FAQs, routers to the broadband hardware database, etc. but it is not easy to remain competitive without some help from the community.
We have a number of people in the IT field that, if willing, can share their expertise with Howto/Articles on Security, SOHO networking, wireless, etc. and post articles on the main site. I'd welcome any regular member willing to step up and add to the information already on the site, technical networking/security articles, reviews, FAQs, etc.
There is already a way for any registered user to contribute articles to the main site:
http://www.speedguide.net/submit.php (choose User Articles, or Broadband stories, etc.). Reviews of networking hardware, howto articles, etc. would help many people, the main focus of the site has always been helping the broadband community.
If you're willing to contribute in any way to the information on the site other than posting in the forums, please feel free to email/PM me anytime and I'd be glad to set it up.
I also plan on improving the router database (info/specs on 265 routers currently) and adding the possibility for users to add to it.
http://www.speedguide.net/broadband-list.php. For now, you can always find your router and add a short review of your experience with it

(there are only a handful of people currently posting to the main site at times).
Historically, many users came here for help with their broadband connection, and became regulars after receiving good information and answers. Helping people in the technical forums is another great way to improve the community.