I've often read that lots of water is a good idea before a show. The one big problem with that, however, is that when I get a bit nervous my need to pee is multiplied ten-fold. Now, this could be middle age, I'll admit, but nevertheless it does pose a bit of a problem. I usually just make sure I've got a bottle of water with me on stage.
I try to make sure to practice, including the day of, so I know things inside and out. I used to use written sheets as a "security blanket" as mentioned above, but I've quit that, because I found that more often than not the paper would draw my eye away, and, its presence suggesting I need such a thing, I found it actually a distraction. So now the only time is if I'm accompanying someone, and I can just focus on the paper.
Before I step up< if I feel the jitters, I tend to do just a zen kind of thing; I imagine something totally unrelated to the moment, something beautiful (horses on a beach seems to work for me; follow their movement, their rythm...), and trust I've done my job and everything will be fine. I had a friend once say that what makes us nervous is our perception of the audience as hostile in some way. But in fact, they're on your side and want you to succeed. (Think how uncomfortable you become when you're watching someone bomb.) And hey, if there is the odd person out there who wants you to fail, what the hell do you care? HE/She must be an asshole anyway, right?
SB