I haven't done a gig yet, still working on my repertoire (and trying to teach myself how to sing and play at the same time).
But when I do vocal performances or karaoke, I try to start with a song that's about feeling confident and/or relaxed. That way, when I start to tighten up I just remind myself to take the advice that I'm singing. For bonus points, I try to pick something easy as well. Jennifer Lopez's "Feeling so Good" seems to work wonders for me (now just to make an acoustic version...
).