If your voice is giving out, the only thing that will really help is resting it - anything that lengthens your session once you start getting tired is likely to make you sound more tired the next day. It's often better just to stop early and have a more productive second day than extend the first day by an hour just to sound bad the next.
if6was9 has covered all the essentials I think.
You have to remember your voice is like any other muscle group - going straight full pelt with it is more likely to cause injury, so you need to warm up. Not only will it make your tone better and let you sing for longer on the day, but if you regularly do it it will improve both over time too. If you look around the internet you can probably find a decent warm-up routine somewhere, but even making "zzz" or "eee" sounds/humming to scales, increasing the pitch of the first note each time for around 20 minutes is better than nothing.
Dairy is bad because of the lactose, it makes your spit thicken up and get 'stickier', which stops your throat getting as raw but wrecks your tone for clean singing, and can make you more glottal-ey (make a "guh" sound before and after drinking milk and you'll see what I mean). Other sugars do the same, but not quite as badly. Chocolate and Cola-type drinks are bad for it.
I'm not so sure about the Vicks suggestion. Vicks is menthol based, so it loosens any phlegm, and I'm not sure that's a good thing right before singing as singing/speaking brings up any crap in your throat - I'd have visions of hacking up during takes, especially if you're a smoker. I've never tried it though, so it's worth a shot! (it won't do any harm)
And I don't think Egan was getting at screamers - screaming with good technique is easier on your throat than clean singing with poor technique. The most important thing is to never force anything. It never helps - making good sounds is all about controlling air pressure, and as soon as you start forcing things you lose that control - not to mention it's an easy way to strain your throat, swell your vocal chords and give yourself a monster headache.
Steve
