If you have to give a presentation, lead a meeting, or just do a lot of talking as part of your job, losing your voice is not option.
The best thing you can do for your voice is to speak only as loud as necessary, with plenty of breath support, at a comfortable pitch. Most people speak at a higher pitch than is right for their voice.
It's easy to forget all of that when you're out with friends at a noisy bar. Whether I've been ill or just been an idiot, I sometimes need to patch up my voice in a hurry to get through whatever's next. I'm neither a doctor nor a vocal coach, but these tricks have served me well over the years:
Drink lemon juice
Not lemonade. Not lemon-flavored vitamin water. Not lemons. Lemon juice. 100% pure, no sugar. It should make your face pucker up just thinking about it.
Keep the bottle handy and drink whenever you start to feel scratchy. Let the lingering taste remind you to take it easy and breathe more.
Drink warm water
As warm as you can stand. Many people use coffee or tea or hot cocoa. This isn't ideal. Caffeine and sugar stress your system, and every little bit of stress is making your voice that much more strained. If youcan't stand drinking plain warm water, try a decaffeinated tea with lemon juice and/or honey.
Avoid cold and dairy
No milk. No ice cream. No frappucinnos and whatnot. While they feel great on a sore throat, dairy and sugar coat your vocal cords in a slime that makes speaking and singing more difficult.
Cold things in general are bad. Many touring band's riders specifically require that the water for the stage be room temperature.
This is hard, especially if being sick is what messed your voice up in the first place. Use cough drops or a warm beverage and try to power through without coughing.
Rumble and hum
Big meeting or presentation today? Phone interview for a job? Hop out of bed humming. Humming is an easy and gentle way to keep your vocal cords warmed up.
Similarly, try rumbling your vocal cords like the creepy kid from The Grudge. Start saying "ahh" and slide your voice down as deep as it will go without forcing it. As you get very very low, your cords should be moving so slowly that the sound becomes intermittent. If that approach doesn't work, make a big, loud, comedic yawn and ride it all the way down. If you get a bit of a rumble at the end, hold it a while. Keep at it until you can just do the rumble.