I usually tune up before I leave home. Take the guitar out and let it acclimatize for half an hour when I get there, then tune it again before going on stage.

If it goes out of tune then, I will give it a quick relative tuning, and sing the notes as I tune "Doh, Me, Soh, Doh". For some reason I do this in G, although I rarely sing in that key. Just seems easier on the guitar.