There also a few directories that list forums in particulare niches. Just do a search for forum directory.
Hey I seen a post somewhere here in Digital point and it had a link, to a blog, that had an amazing list of forums but.... I don't recall where it is. go to here and you will find many, http://www.yawoop.com/search.php?page=0&query=Advertising+Forums Digital Point always comes up first at yawoop hahaha (also remember this forums list has the niche or search term Advertising you can change that accordingly!)
http://www.digitalbattlegrounds.com/ i ran across this blog today that has some kind of forum list happening for differe things
That's the way I do it. That is the best way I find as I just hate some of the forums software and prefer to join the ones that are more user friendly, for example, that quick reply box down the bottom of each thread is very useful, but not all forums offer that I don't think. So "powered by vbulletin" +keyword If you use a phrase and not a keyword then also include quotes around your phrase. You may also want to put a + sign before the start quotes of both the phrases aswell like: +"powered by vbulletin" +"your phrase", but not sure if the +'s are needed when they are surrounded by quotes.