Calming the Nerves
Practice
Experience
Work Up to the Big Gig
Drugs and Alcohol
You may be the kind of guy that likes a cocktail or a toke to calm the nerves. Cool. If it works, I recommend doing so (of course I don't recommend becoming an addict spiraling into a life-destroying tailspin). The problem is that intoxication often has adverse effect on your playing ability. It can also disorient your perception to make you think that your playing is better or unaffected when intoxicated.
If this is going to be your method to deal with nerves, you have to be sure it won't hurt your performance. Plan ahead - try getting intoxicated when you practice, playing through entire songs as you would during a gig. Record yourself on a phone or video camera and verify that your performance is as good as you thought it was.
Ultimately, I recommend experience, preparation, and mental exercises over this approach; however, it can assist get over the initial hump for the absolute novice whose nerves would otherwise be far worse for the performance. Once gaining experience, however, you should transition away from it.
