I have a very demanding vocal schedule, so the way I keep the illnesses, not to mention the bad singing at bay, is 3 liters of water everyday. Keep in mind, I have a very small frame (5'1.5"), so if you are taller than me, consider maybe more. Check with your doctor to get a good amount, but you do not want to be drinking so much that you're drowning yourself from the inside.
There are these great longezs called "Slippery Elms" which are AMAZING. They have a weird feeling, but if you stick one in a half hour before a show, they are sure to improve your vocal tone considerably.