Solving Feedback and Distortion

Feedback (squealing or howling)

Feedback happens when a microphone picks up its own sound from the speakers and re-amplifies it.

Turn down the channel that's feeding back first, then find the cause.
Keep main speakers in front of the performers and pointed away from the stage. Microphones shouldn't be in front of the speakers.
Turn stage monitors down, or move them so they don't point at body mics.
Only open mics that are in use: mute the channels of performers who aren't speaking or singing.
Check mic placement: a mic closer to the mouth needs less gain. See Microphone placement.
Use the mixer's EQ to reduce the specific frequency that rings, if you know how. Otherwise lower the overall level.

Check the transmitter gain/sensitivity. If the receiver's audio peak light flashes when the performer sings loudly, turn the transmitter gain down.
Check the mixer input gain isn't clipping.
Check the receiver output level matches the mixer input (mic vs. line level).
Fresh batteries: a weak battery can cause distortion on some systems.
If only one mic distorts, swap the mic to another pack. A damaged capsule can sound distorted or muffled.