Start with things like "applause" "awwwww" and "*wolf whisle"

work up to "too soon!" "you suck!" "*gasp*" etc.

Can restrict to a few cards to cover the scene. Perhaps have different sections of the audience be given different cues due to an argument between the sign holders.

eg. there is a male and female sign holder. Generic attractive female comes onstage, male sign holder instructs crowd to "wolf whistle" female sign holder is discussed and instructs her side to boo.

or. one character hits another. One side cheers, the other boos.