not a good idea to have caffeine before a performance. it might help you, but i would advise others to stay away from advice like that. if you are very nervous, caffeine will just worsen you. also, if you have things like : a quick coffee, a shot of whisky, or wine etc. you may start to become dependant on those things before the performance. to the guy who said he has a coffee to wake up, if you are not alert for your performance without coffee, then i would be very worried.
basically, putting your hands under hot water will not work on its own. you need, like the others said, to combine it with some flapping of the hands, arms and jogging on the spot etc.