Warm up. This cannot be stressed enough. You should never ever sing cold. Warm up your entire range with scales before each concert.
Sip water during breaks in the performance. This will keep your throat hydrated. Make sure that the water is at room temperature. Cold water will tighten your vocal chords and could cause your voice to crack. Warm tea is also great to drink in addition to water.
Avoid sugar. Sugar will make you thirsty. You want to stay hydrated, so anything that makes you thirsty is just not a good idea. Try to eat your meals at least two hours before the performance so your body can digest. You don't want to perform right after you eat and risk your body having an unpredictable reaction to the food you just ate.
Gargle with saltwater. If you are experiencing any soreness in your throat, gargling with saltwater can really help. It may be enough to make you comfortably survive the performance. However, if the problem persists you should see a doctor immediately.
Wear a turtleneck. If it is cold where you are performing, wear a turtleneck when not performing. This will keep your throat warm and protected from the elements.
Use a humidifier. Put a humidifier in the room where you sleep. Dry air will irritate your voice. Know that 30 to 50 percent humidity is ideal.
Exercise. Exercise or yoga can really go a long way in aiding your singing. Anything that keeps your mind and body relaxed will also help keep you more confident and natural when performing. It will also build your stamina, so that your body is more equipped to endure a rigorous performance schedule.