Here's something that every live engineer runs into and it's a constant struggle to deal with it. Singers cupping the mic. It's the bane of our existence. You get some hot shot on stage that wants to look like some star he saw on TV, cups the mic, creates a feedback nightmare, and then demands more vocal in the monitors.
Pretty much the only way to put a stop to it is to just be that brutal sound guy and say something like, "You can cup the mic, or you can have vocals in the monitors." But that's not the kind of service we want to give is it? My technique has been to try and educate people when time permits. It shows them that I'm not just another jerk sound guy but that I actually care about their show, both on and off stage. So how do you go about educating the masses? I have a little physics lesson that I lay on 'em. It takes less than a minute and more often than not it works.
The reason a mic has a handle on it is to allow the head of the mic to function properly. There's a lot more going on up there than just sound going in the end of the thing. The diaphragm has two sides. The side you sing into hears your voice, plus all the other noise coming in from the stage. The back side hears mostly just the noise. So when the back side noise presses against the diaphragm in the opposite direction of the noise from the front side, they mechanically cancel out and (mostly) just your voice comes through. When you cup the mic, the more you cut off the back side from "breathing", the more omnidirectional the pattern of the mic becomes. More noise, higher likelihood of feedback, and cruddier sounding vocals all around.
After I drop science on em I'll usually give a couple relevant examples of famous people who properly use the handle that the manufacturers have given them. I stress the point once again that I want them to sound good, have good monitors, and a good show in general. Then I'll offer to put a little strip of gaff tape on that sucker to act as a little warning track. That way they don't have to think about it. As long as they can feel that little bit of texture with their thumb and forefinger they're good to go.
Does it work? Yes. Every time? No. Sometimes you have to get the guy right in front of a wedge and show him the difference between cupping and proper holding. Even then some of them can't hear it or just don't care. By and large though educating the masses is possible, and if you're well rehearsed with your spiel you can drop knowledge on a singer while you're doing a changeover without missing a beat.
Here's a little diagram to better show what I'm talking about. The number of dashes indicates the strength of the signal coming from a given source or direction. If the noise is allowed to push equally on both sides of the diaphragm, it cancels out mechanically before it ever gets a chance to generate any signal into your system.
(handle side) Diaphragm (grille side)
noise ===> | <=== noise
| <====== voice