This extract is taken from SOS:
Some singers just can't work with headphones, and in this case, the only alternative is to use loudspeakers for monitoring. This obviously raises the question of spill, but in situations where a modest degree of spill isn't a problem, you can get away with keeping the mic fairly close to the vocalist and setting up a speaker (or pair of speakers) behind the mic, in much the same way as you would live. You'll need to use either a cardioid or hypercardioid mic to make this work properly, and the monitor needs to be positioned in the 'dead' area of the mic, where it picks up little or no sound. It is also important to position an absorbent surface behind the singer, to minimise the amount of sound reflected back into the 'live' side of the mic. Even after you've taken these precautions, the monitoring level should still be kept as low as possible.
A more effective solution is to use the 'out-of-phase' monitoring trick, which involves sending an identical mono foldback mix to two speakers, one of which is wired out of phase with the other. If a microphone is placed anywhere on an imaginary line equidistant from the two speakers, the sound fields will largely cancel each other out, resulting in very little spill. The mic position is critical, and the best way to do the final adjustments is to listen to the mic channel using headphones while you play the backing track. As you move the mic, you'll hear the spill level change, and it doesn't take long to find the spot where there's minimum sound.