I fall into the category of I will sing if someone else has started it.
At away games its so easy to join in.
At home, I end up waiting for a song to start and by the time it does, you end up singing a couple of lines.
My suggestion is small speakers under each seat. we then have a kareoke stand with disco ball in the corner stand and people take turns to sing, the speakers aren't loud enough to project the sound but they let everyone feel like they can join in.
Bonus points for tribute acts, slutty dance routines and celebrity performers