The answer is utilize all of them, I always make the analogy that it's like getting free rent. If you were offered free rent to open a store and sell your products, how many stores would you open? As many as you could right? Then do the same online, open as much "locations" as you have time for, introduce yourself, follow the replies and build up your name within the industry.
You can join webmastersgossip.com and if you write blogs you can have your own blog on our forum too. You can see the list of our member's blog here
Yah I would have to say that Big Boards is one of my personal favorites for finding the bigger forums via niche. --Thanks!