Getting a singer to lose their inhibitions

Telling them that they'll be match EQ'ed to sound like Britney Spears by Mr.Kent if they don't deliver the br00tz could help too.
Fear of loosing ballz -> awesome motivator!

But on a more serious note:
That really depends on the vocalist...for me it helps to get myself into a "live" mood & to try to not focus too much on how it should sound/how one wants it to sound.