Knocking a couple back before a show is not a big thing.
Getting fucked up just so you can get on stage is a bad thing.
The big thing is confidence. If you have confidence in your playing abilities, then make sure everyone in the crowd knows that. The crowd is like a woman, they know when you're nervous and when they see that they don't give a shit about ya.
When you get yourself ready for a show, just keep telling yourself you're the best musician out there and after the show you know that everyone in the crowd will know that. Even if you are self-concsious about yourself, or playing onstage, NEVER LET THE CROWD KNOW IT.
Rock hard, have fun, and play with a chip on your shoulder. But not too much of one though.