What worked was to put a 1 day time limit on management messages, and a 200 word limit. If they wanted to keep a message up two days they had to type it in again the next morning.
Also, we put a random "fortune cookie" text on the status page that changed at every reload, under the "management message" area. That encouraged people to actually read the words under the status numbers.
Everybody but management had a very positive response to running the "management message" bit through `cowsay`; too. Dunno if the effect would have lasted, but consider it for things that are important.