I started going to our local song circle because I was getting frustrated wishing I could sing in public but never having either opportunity or nerves to do it. My confidence is growing, slowly, but I'm always and inevitably nervous in public, I'm just getting used to singing through it. I usually figure the first time I sing a song in public I'm likely to screw it up. The local song circle is very useful in this instance because if I have a gig I'll sing a new song there first and get that first time performance over with. Apart from that its practice, practice, practice. I tried using a drink to relax myself before a performance but although I was relaxed I also sang worse so I prefer not to drink until afterwards. I also spend a good length of time warming up before performing.