[jackweasel]

In addition to all the great advise given above, I would like to add this suggestion. If you have your own p.a./ sound system, try to set your rehersal place where you can practice through it. Sometimes the mic itself can be intimidating, but in my opinion, getting used to your voice though a sound system and learning how to use it is as much a part of putting on a show as being a good guitar player or singer. It's the final link in the signal chain between you and your audience. Be as comfortable as you can with all the tools you use to do your job, and it'll come through as a better performance.
Recently I played a family reunion for some friends [I rarely "gig" these days] and afterwards, my wife gave me a huge compliment on my performance [something she NEVER does] and said, " I can tell you've been practicing a lot with your Bose [L-1] and all your other stuff. It sounded great. You looked like you were having a good time, too."
That's exactly what I needed to hear.