I've always had it, crippling some days, its never gone away. Actually getting on stage and playing sometimes provides relief but not always. Only thing I've found that helps is a mint tucked in my cheek to help with the dry mouth, which isn't so good if I'm singing.
For me, its about baring my soul up there in front of all those people, matters not how well prepared I am.
I do find that the bigger the gig, the less anxious I get. Which seems to be the wrong way round doesn't it