Avoiding feedback
Acoustic feedback will occur when the microphone picks up audio from the loudspeaker and transmits it back into the amplifier. A "loop" of sound occurs which builds in intensity to a piercing scream. This is usually due to the volume being too high or the microphone too near to the equipment - and the scream won't stop until you physically break the loop of sound!
Turn it down!
Every venue has a point where feedback can not be avoided.
Is the singer using the microphone correctly?
 If the singer is holding the microphone too far away from his mouth then you may have to turn the PA system up louder to compensate and then you will get more feedback. Also, the singer should not be touching the wire at the top of the microphone otherwise the vents that create the cardioid pickup pattern get covered and this causes foldback.
Are you using cheap microphones?

Cheap microphones are more prone to feedback.

Are your speakers in the right position?

Are your speakers positioned far enough in front of the line of your microphones? Try moving them forward a little at a time. If the singer says he can't hear, consider getting stage monitors. Try angling the PA speakers differently Even slightly changing the angle of the speakers can affect feedback enormously.

Isolate which microphone is causing the problems and change the EQ settings Feedback occurs at varying frequencies. You may be able to adjust the EQ on the mixer to cut down the feedback.

Are you using the right type of microphones?

Omni-directional microphones pick up as much sound from behind (unwanted sound) as they do from the front. Cardioid, Super Cardioid, Rifle and especially Microline pickup patterns are much better for stage use though you should be aware of the drawbacks with each type. For example, Super Cardioid pickup patterns might reject sound from the back very effectively but they are also very susceptible to handling noise and tend to emphasise "P" and "B" consonants explosively through the sound system, especially when used close to the mouth.

What are the natural room acoustics like?

Establish what the natural room acoustics are like - try shouting loudly or clapping. If there is a basic echo in the room then you may be fighting a losing battle. If there are curtains or drapes, try closing them to reduce the amount of natural reflections you get. Feedback at sound check time When you do your sound check, the room will be empty and cooler than when you actually play. Bodies absorb sound and cold rooms make feedback worse. You may therefore be able to cope without the feedback when you are actually on stage.

Using wireless microphones?

These are more prone to feedback as the singer is free to move around the stage more and can move in front of the speakers. Use the sound check to establish their 'working area'.

 

Click here for an excellent Sound On Sound article "Getting Your Studio Sound Live" which goes into more detail about positioning your equipment on stage and the causes of feedback.