Plain water is the most suitable drink to have to keep your voice hydrated. Because it contains no additives or preservatives, you get no unwanted side-effects. However, ice-water is not suitable as the temperature drop causes your throat muscles to contract, which inhibits your ability to sing. Room-temperature or luke-warm water is the most suitable. Sparkling water is fine too, but no less than an hour before performing as it can give you gas.
The Texas Voice Center recommend using baking soda as a throat gargle (See References 2). Mix 1/2-tsp baking soda, 1/2-tsp salt and 1/2-tsp corn syrup with 6-oz of warm water and gargle gently. This solution is especially useful if you've just flown into town for a show and the plane's on-board air-conditioner has dried out your throat.
Honey has a smoothing effect on your voice. Mix a 1/2-tsp of honey in with a glass of luke-warm water before you begin your performance. Using local honey is good if you suffer from pollen-based allergies because it builds up your tolerance to the local pollen. As allergies can of course affect your voice, any advantage you can get to avoid them is to your benefit.
Drinking fresh, pure fruit juice, such as pineapple, orange and apple juice, helps you maintain a high level of energy and generally helps your immune system. Don't drink acidic fruit juice such as orange or grapefruit juice any less than an hour before you're due to sing. The citric acid can cause mucus to develop in the throat.
Any substance that is a diuretic is unsuitable for a singer. Such substances include coffee, tea, caffeinated soda and alcohol. Hot drinks, provided they contain to no caffeine, are fine up to an hour before performance. Milk and any drink containing milk is unsuitable because the lactic acid causes mucus to form in throat.