Adjust output volume of the speakers. Feedback can sometimes be avoided by simply lowering the overall sound output. If a high volume is needed, this solution may not be ideal.
Position microphones so that they are not pointed directly at a speaker. Alternatively, reposition speakers so that they are not outputting signal into the microphone path. It is a good idea to position the speakers in front of and pointing away from the microphones.
Angle speakers so they do not point directly at a flat surface such as a wall. This will help stop sound waves from bouncing back into the microphone's path.
Utilize microphones with a unidirectional pick-up pattern, and use fewer mics when possible. This will ensure that only the sounds desired in the microphone feeds will be picked up.
Use the equalization controls on your mixing board to cut out certain frequencies that may be causing feedback. It may take some trial and error to determine which frequencies these are, so simply adjust the EQ knobs gradually until feedback is reduced or eliminated.
Employ a noise filter or feedback eliminating device to automatically locate and reduce feedback.