How do I stop my microphone from feeding back?

Feedback occurs when a microphone picks up the amplified output of its own signal through a loudspeaker, and that loop gain exceeds 1 — at which point the system oscillates. The primary controls are the microphone polar pattern, gain structure, and physical positioning. A cardioid microphone pointed away from the loudspeakers has the greatest rejection of their output. Lower console gain means less amplification in the loop. A microphone held closer to the source requires less gain — reducing all gains simultaneously. The relationship between microphone-to-source distance and gain before feedback is direct: halving the distance between the microphone and the vocalist's mouth reduces the required input gain by approximately 6 dB. That 6 dB allows 6 dB more headroom before the loop reaches oscillation. Performers who pull the microphone away from their face while singing are giving away this headroom unnecessarily. Physical positioning of loudspeakers relative to microphones is the most powerful feedback control. A main PA positioned in front of the microphones, aimed away from the stage, is fundamentally better than any configuration where loudspeakers are behind or beside open microphones. No amount of EQ corrects a geometrically bad speaker placement.