Drink plenty of fluids [6-8 glasses daily] not including coffee/strong tea/coke/alcohol and caffeinated sports drinks.
Sip water when making presentations.
Avoid dairy products if you suspect that they cause an increase in mucus.
Minimise Overuse/Misuse of Your Voice
Do not smoke as smoke is a major irritant to the vocal cords. Also avoid fumes, pollutants, dust including chalk. Where possible wear a mask when using chemicals, sanding surfaces etc.
Avoid excessive alcohol. For every alcoholic drink you have drink a glass of water.
Do not clear your throat or cough habitually. Instead swallow slowly with or without water; or use an air cough (a singer's cough) that is a huff to clear mucus from the vocal cords. Only if absolutely necessary give a single, short, gentle cough. Throat clearing/coughing damages the larynx owing to the hard contact of the vocal cords.
Avoid talking over background noise [music, parties, machines, etc] or over a long distance. Lessen the distance between you and the listener. Only speak to those within touching distance. If noise is excessive and you have to talk keep talking to absolute minimum, planning to communicate more fully outside the noisy environment.
Avoid shouting, screaming and whispering.
Do not speak to a large audience without a microphone.
Avoid using your voice too loudly, or for too long.
Take a regular break from using your voice. For example, teachers should avoid noisy staff rooms between teaching sessions; and singers should avoid socialising between performances. Try to rest your voice for 15 minutes of every hour.
Ensure your voice is not monotone. Use variety in the tone of your voice.
Avoid singing outside of your range (either too low or too high).
Avoid speaking beyond your normal breath support. Speaking more slowly may help this.
When you have a cold / sore throat / laryngitis:
Don't use your voice more than absolutely necessary, but do not whisper. A gentle, quiet voice is better for your larynx.
Cancel any non-essential commitments.
Inhale steam twice a day.
Drink plenty of fluids.
Drink hot beverages to soothe the throat.
Chew sugar free gum or sugar free lollies, rather than medicated lozenges.