All very good advice here. I’ll just add one tidbit.
The admonition against over-practicing is valid. You should definitely do some trial runs in your “performance” voice, so you can figure out the best places to breathe and whatnot, but if you’re not using your voice properly, as GreenEyes said, you can really screw yourself up.
One of the most common problems with vocal use is placement. Your head is a resonator, and you can use that. However, for a variety of reasons (“manly” voices are deep, “feminine” voices are high and thin, etc.), a lot of people shift their resonance to their throat, their chest, or whatever. This is fine for everyday speech, but if you try sustained singing or oratory, you won’t sound so good, and you can even hurt yourself.
How do you place your voice properly? In my experience, there’s a very simple trick that will work in the short term. When your voice is correctly placed, humming will make your face vibrate. It’s called “mask resonance,” and it’s the most efficient means of getting maximum vocal quality with minimum effort and risk.
Try this. Close your lips, and open your jaw. Not too wide; pretend you have a plum in your mouth. Now hum, at about speech volume. The place that vibrates the most is where you’ve got your primary resonance. It’s impossible to explain textually how to shift this, but just fiddle with it. Don’t change your pitch (high or low), just try moving your jaw, lowering your Adam’s apple, shifting the back of your tongue, or even stroking your temples with your fingertips to psychologically suggest pulling your voice upward. By experimenting, you should figure out pretty quickly how to shift your resonance area. Then, once you’ve got your face vibrating, try opening your lips briefly, saying “mmmmmma ma ma ma mmmmma ma ma.” If you’re lucky, and your voice is already resonating efficiently, this won’t sound so different. However, it’s more probable that you’re like the majority of people whose voices aren’t resonating as well as they can, and you’ll be startled at how your voice “pops” out of your head.
Try it out for a while, and see how long you can sustain the effect. It takes a lot of training and practice to move your voice to a better place permanently, but if you do this a few minutes before you perform, the effect should last long enough to get you through the song.
Hope this helps.