I've got a new way for forum owners to get exposure to members of other forums, people who are already active using forums and enjoy them. It works on a forum points system, so your forum has to currently offer points to it's members.
My game site http://www.ultimategamester.com is a site where I run points contests on behalf of forum owners. If you go there you will see an example of a forum contest that was recently set up.
The more forums that get involved, the more each forum benefits from the cross-pollenation of members with similar interests. All contests will be listed on the home page of my site, and people from all forums will see the other forums' contests.
This all takes no administration by you, the forum owners as I will set up and run all contests. If you are interested, all I need is a supply of points from your forum (after I join) to award your members who win the contests.
Well, if members of forum A come to the game site to take part in the contest for their forum to win points (announced in a post), then they will see the contests for forums B and C and so on. All of the contests are named something like "YourForum.com Points Contest".
So, there is exposure to the other forums. All of the visitors to the game site are forum users with common interests in forums, games, and points. They are the people you are trying to reach without fishing for members on forums.
The more forums that participate, the more it benefits everyone.