Drink water regularly to keep your vocal folds, mouth, throat and lungs in working order.
In addition to drinking water, adding humidity to your environment prevents throat dryness, which can cause undue stress on your vocal folds and affect the shape of your voice.
Limit your volume. By keeping your voice at a reasonable level, you don't place additional strain on your voice, other than what is needed.
Rest your voice often, especially after extended periods of talking.
Inhale as if you are yawning, with the tongue pressed to the bottom of your mouth and the jaw, shoulders, neck and head relaxed. When you exhale, make a sound with your throat. Do this several times, each time changing the pitch. This keeps your vocal folds in shape.
Vocalize an "m" sound in a comfortable, low pitch. Hold this for two or three seconds. Without stopping the sound, open your mouth slightly to create an "ah" sound. This helps strengthen the vocal folds for producing lower pitches.
Place your lips together and blow through them, vibrating your lips and creating a buzzing sound. Slowly move up and down in pitch. This may cause your nose to tickle at lower pitches.