Okay, here's the situation I have:

My band's got their first gig tomorrow, we had our last practice before the gig tonight but the singer didn't turn up as he's had food poisoning, which has prompted him to say he "might" come down to the gig tomorrow.
This has left us screwed basically as the band we're supporting have said if we don't have a singer we're not going on pretty much, which has left us as needing to sing for ourselves, to find another singer (not possible) or get some people to do it Karaoke style. I've asked some people and noone's willing to do it so that leaves us needing to sing for ourselves.

Only really three of the songs can be sang by us, two of which by me, anyway, herein lies the question.
I've over the last year been working on my vocals and I'm much better than I was, but I've been a bit negligent on my voice as of recent so it's gone a bit pear-shaped as of recent.
Are there any excersises or techniques that'll help me sort of, bring my voice "back" so to speak?

Also, I've noticed that when I sing in a mic the tone of my voice seems to have way too much high-end, even when I'm trying to emphasise the lower register, is there any way to change this? (bearing in mind I probably won't be able to mess with the mic's EQ as it's the headlining band's gear and I don't think they'd be coolw ith us messing with their stuff).

I'll be practicing the songs I'm doing like mad from now up until the gig but what I'm really after is anything that can warm me up back to my standard of a few months ago.

Man, drink hot water, get plenty of sleep, if possible even take a small nap before the concert, because your throat always wakes up better. try to boil some water and breathe it in, don't speak too much, drink some tea.
I'll do that, thanks!

I'll tell the bassist to do the same

There's only the slight problem in that I need to help the band we're supporting set up gear and shift their P.A, but I'll see if there's a sofa in the venue I can rest on.
don't sleep before you sing, that's a terrible idea. Your vocal folds fill with fluid when you lay down, which impedes singing. Speaking at a normal volume is fine too, as the more air that passes by your vocal folds will help them warm up and the fluid to drain.

Setting up their stuff will be good for you. Doing work gets your heart pumping, your blood moving, and your breathing heavier which all help to warm up your voice without even having to sing.

Lip trill warm ups are great to. Try some do-re-mi-fa-so-fa-me-re-do warmups using that technique
Ah okay cheers.

I'll be doing those sort of warm ups before the gig definitely.

Though I think the bassist is prefering to cancel the whole gig over having us sing even though I can guarantee half the crowd are going to be coming for us >: (

Damn singers.

But still thanks for the help I'll be doing all of this if I can get the band to be willing to go ahead with it.