Start with some deep breathing, then mouth stretching and yawning to warm up. The latter increases air-intake and relaxes the muscles at the back of the throat. Move on to humming, starting low and working your way up through the registers.
Take regular rests. They needn't be long disruptive rests, but you should never use your voice constantly without stopping. If you're a singer, plan instrumentals for the live set so you get a chance to rest your voice. If you're a public speaker, prepare a brief visual presentation to give yourself a chance to stop. Even short rests for a sip of water help to prevent voice loss.
Stir a teaspoon of honey into a cup of hot water, leave it to cool and then sip it in between periods of voice use. If possible, use locally-produced honey. This increases your tolerance to pollen related allergies, a cause of vocal irritation and potential cause of voice loss.
Project within your capabilities. Everybody has a natural limit, but it's only natural to strive for those high notes or to speak for longer periods of time, especially if you're feeling good and your audience are responding. If your throat feels tight or sore, don't push on because you could be out of action for the following night's engagement.