One idea which has already been mentioned is to hold an old skool night eg old skool garage, dance, drum n bass etc.

Battle of the DJ's night and the winner could maybe have a regular slot in your club

Once a week hold a buy one get one free night on selected drinks.

Free entry before 11 or somthing or maybe just for woman.

Once a month hold a proper night with good big Dj's

Although i hate these night....skool disco

70's,80's nights

Crazy dress nights

Sambuca night With sexi girls giving out free booca's all night