A voice over talent's main instrument is their voice.
Like any artist, without proper tools you can not create.
Taking care of your voice should be always at the fore-front of your mind.
Vocal health is a large part of any voice over talents day to day routine.
According the the NIDCD (National Institute on Deafness and Other Communication Disorders) They offer several good tips on keeping your voice healthy:
Stay hydrated, like a well oiled machine your voice needs to sty lubricated, they recommend 6-8 glasses of water daily.
Limit alcohol and caffeine, they help the body to lose water making your vocal folds and larynx dry. (alcohol also irritates the mucous membranes that line your throat).
In drier climates or during the winter, use a humidifier in your home to add the moisture back into the air. try to shoot for at least 30% humidity.
Try to avoid or limit medications that dry out the vocal folds (some allergy and common cold medicines do just that). Always ask your doctor which medication(s) would be best for you.
Don't smoke, and try to avoid second hand smoke.
Eating a healthier diet of grains, fruits and veggies can help your voice! (Vitamins A, C, and E help keep the mucus membranes in your throat healthy).
Exercise regularly, it helps promote good posture and breathing.
Avoid mouthwash/rinse containing alcohol (see #2) if necessary, try a warm salt water solution instead.
Avoid speaking when your voice is tired. Like any muscle your voice needs rest.
Practice good technique, use that diaphragm, support your voice, don't rely on your throat alone.
So treat your voice well, remember these tips, and in return your voice will thank you by making you sound your best!