My singing teacher used to tell me not to drink any milk, even in tea, before singing as it increases the production of mucus in the throat. Drinking coffe just before a performance tightens the throat and really cold water also closes things up. I drink a throat medicine tea that has licorice in it and find it very useful for soothing the throat and stopping it from feeling too tired out. Alcohol muddles things but also, in small quantities, helps me get over nerves so I try and figure out if I need it any particular time. If your'e going to be singing in an evening its a good idea to make sure you stay hydrated throughout the entire day leading up to it so your throat will be loose and relaxed.