I find that alcohol is counterproductive and can cause the very mistakes/miscues that I might be nervous about.
I do my best to prepare: rehearsal, knowing the material if I'm filling in, and investing in my own musical knowledge/chops/technique/etc so I'm as comfortable as I can be no matter what happens. But, the most important thing for me is to remember that I'm a perfectionist--but music is a living, breathing, and inherently imperfect thing. We've had one or two major situations, where the guitarist started playing a different song than the rest of the band--but we know our following so well that we can stop the song, laugh about it with the audience for a moment, and then move on.
Great example--I was at the Taylor Swift concert in DC last night (cough with my daughters). She got onto this apparatus that was supposed to lift her up high over the crowd, but it jammed after only lifting up a few feet. It wouldn't go up, and it wouldn't go down--a MAJOR problem for such a scripted production as hers. So she was stuck on this thing with only her microphone and her guitar, and she made it look completely easy. She was a total pro--she played a song on the guitar, then asked over the main PA for her roadies to set up her keyboard on the platform and she played a couple more songs, still stuck up there. She laughed and joked about how her crew was telling her in her IEM's that she might be stuck up there forever. She turned what could have been a total disaster into a great moment just by being relaxed and engaged with her crowd.
So for me the answer is just being loose, accepting imperfection, and being connected to the crowd.
Tom
PS I bet Taylor Swift had some choice words for her road crew later tho....