get a big banner and drape it over doorways in the front of halls n such
thats what they do here
oh, and you could also find out where the biggest classrooms are and ask the prof if u can announce prior to the class about the lan. we have a class of 2,700 ppl and there are usually 2-4 groups per class that propagate their events, but even in the smaller 100-300 ppl range they do it too