Eventually every singer has some vocal trouble, and if you’re not careful, it can really lead to long term damage. That’s why it’s important for a singer to learn to be especially aware of the need to take care of him or herself.
Here's an excerpt from the band improvement book How To Make Your Band Sound Great with 7 tips to help vocalists keep their voices healthy and ready to sing at every gig and recording session.
"1 - Aside from being sick, the number one cause of vocal problems is not getting enough sleep. When you’re tired, all the parts of your body needed to support your vocal cords tend to weaken a bit, which leads improper breathing and thus throat problems shortly after you begin to sing. Get as much sleep as you can (preferably seven or eight hours) the night before a gig, or at least take a nap on the day of the gig so you can feel somewhat refreshed.
2 - The next thing is to avoid milk (and any dairy products for that matter) from three to six hours before you sing. Anything with milk in it will cause an excess production of phlegm around your vocal chords, so that’s a definite no-no. The old remedy of milk and honey for a rough throat is very soothing after the gig, but not before!
3 - If you are hungry before a gig, don’t be afraid to eat, but just eat until you’re satisfied and don't stuff yourself with a seven course meal. Try not to eat in the last hour before your performance in order to avoid that excess phlegm again. If you do feel phlegmy, you’ll have the strongest temptation to clear your throat (which can be harmful) immediately after eating, but waiting an hour is usually enough time for your meal to settle.
4 - And speaking of clearing your throat, there are some that say that you should never try to clear your throat because it can cause some damage, but it’s usually necessary because excess mucous inhibits really inhibits your singing. The trick is to find a way to clear your throat without irritating it and the best way is to do a gentle "whispered cough" and then swallow and repeat. If this doesn't work, you need to deal with the excess mucous production. Squeeze a 1/4 of a lemon into a tall glass of water and sip over a period of about twenty minutes. This should cut through a lot of the excess mucous.
5 - Other things to avoid are alcohol, tea (despite popular belief), coffee, cola and anything else with caffeine, since these actually have a dehydrating effect, which is quite the opposite of what you really need.
6 - One thing you should do is drink lots and lots of water (ideally two to three quarts a day - the more the better) because a dry throat leads to a sore throat. If you live in an arid climate like Arizona, sleep with a humidifier next to your bed and try to warm up your voice in the shower. The moisture can be an incredible help for your voice. Also, learn to breathe in through your nose as much as possible. This will help moisten the air before it reaches your vocal cords.
7 - Finally, some singers swear by Entertainer’s Secret, a spray mixture that lubricates the vocal cords and was developed by an ear, nose and throat specialist. Others really like Superior Vocal Health's Throat Saver, a completely organic vocal lubrication, also developed by a singer."