I either listen and try to pick it up (I've got a good ear for picking up tunes), the words take some time and make for some great 'mondegreens'..... or else I drink my beer... gives my mouth something to do that makes it obvious that I'm not singing because I don't want to, not because I don't know the words!

There is no law that says you must sing along. No-one will report you to the Chorus Cops if you don't sing every single chorus.

LTS