i could tell ya but then i would have to ...... tell ya
PR is the way to go... for example when im out of town i advertise my clan and BFP4F on hotel computers using screenshot and note folders named as such in hopes others will open them and take a look and vist the site(s).
you could also make a signature and use it in other forums as a link back to here when its clicked on.
but what do i know im a PvP'r not much into to co-op(but im giving it a go)... just find those co-op places and do some PR like in Off Topic as long as forum rues are not breached / broken.
you need a catch or a hook rather...something new , different, eye catching, unique, or just plain out in the open and up front and personal so to speak.... which is the most difficult way to go and takes a bit of dedication to stand it strong....if that made any sense.