I always tell quiet singers - Kiss the mic! Seriously, I want your lips touching it!
Which is a technique I use as well, but you'll still need to deal with those that cup, those that hold the mic sideways, sing 2' from the mic, point the mic at the monitors,etc. One thing I do has already been mentioned, turn down the monitor.
One other thing I do that has worked very well on a noisey stage is to use a mic with good side and back rejection (PR-35 or equal), and turn/move the monitor(s) more to the side of the vocalist. Even a slight change from a straight back position will help.