Presently, I am reading a great book entitled, "How to position yourself as the obvious expert" by Elsom and Mark Eldridge (If you are a coach or consultant, it is a must read!).  The book contains some helpful tips on ways to prepare your voice prior to giving an interview to the media.  This would also apply when giving a presentation at work:

"Drink water at room temperature.  Do not drink caffeine, orange juice, iced, or carbonated beverages.  Caffeine and cold beverages constrict your vocal chords.  Orange juice is acidic and traumatizes your vocal chords.  Dairy products make you want to clear your throat, and carbonated beverages make you burp.  Just prior to the interview [or presentation], hum loudly for 3-4 minutes.  This warms your vocal chords and enables you to sound as confident in the first 30 seconds as you will later in the interview [or presentation]."

What else have you found to be helpful to prepare for a speech, interview, etc.?