For those types of things, alternate communication channels work best for us. Call people up and let them know. Instant messaging, twitter, or even email could work too, but I don't think as well. Problem is, people get into a groove, and it really is tough to get out of that.By announcements I mean "This cool thing is happening, you should check it out" - not "If you post pictures of tubgirl, you'll be shot in the foot"
I would reserve the software tricks for important things like major changes in terms of service.
Although I have seen things done like locking innocent threads (everyone reads locked threads, don't they?) and putting the message at the end... things like that only work 1 or 2 times, and people are generally not happy about that.