The first thing I do when I show up to play at a venue is make friends with the PA person. I've seen musicians offer abuse to the sound engineer and I've always thought it was the dumbest thing to do just before someone has your sound in his hands.

An old friend played in a rock band and usually used their own PA. He also did harmony vocals with a harmony machine. One show they had a singer-songwriter as an opening act who heaped abuse on them about the PA system. Lots of mean-spirited, foul abuse. Just before he started his first song, my friend pitch-shifted his vocal mike up by a quarter step. Instant karma!