Here's one that's been around forever but with better mics coming out all the time it gets used less and less. Still, it's not a bad one to have in your arsenal should the need arise.
If you've got a quiet singer on a loud stage, there's only so much you can do. Mics will only reject so much noise and there are limits to how much you can EQ the monitors. If you just can't get that quiet vocal to stand out loud and proud here's what you do.
Get a second, identical mic and put it on the same stand. Set them up exactly the same. Same trim, same EQ, same routing, same fader position. Then you flip polarity on one of them, and have them sing into the other. One mic has the singer plus all the bleed from the stage and the other one just has the bleed. By inverting the polarity, if everything is dialed in correctly the noise should cancel out and you're left with just the vocal.
Now you might think to just put a hypercardioid mic on the case, but not every singer is up to the task of staying right on mic. This gives you a little leeway. It also doesn't work if they're going to rock it hand held, but this issue crops up more with back up singers than the lead. Chances are you've got a pair of 58s in the box you could try it out with. All you need is a dual mount or a drum claw and you're off to the races.
Well Brethren of the Knob and Fader, that's is for this edition of Mic Trick Week. Get those requests in and when we've got another batch we'll do this again.