How do you save the guitar on rock band?

First, your yellow energy/performance meter has to be filled. If you're the:

Microphone: You scream ("Wooo!!" , "Yeaah!!" , etc.) into the mic. to activate it.

Guitar: Jerk the neck upwards.

Drums: They do it by themselves.

When you do this, the guitar (or any other instrument) will be brought back in with the Word SAVIOR or SAVED behind them. ... Unfortunately, I think this will only work if you're in Multiplayer mode. :)