Cut back on the number of mics you have on stage. The more microphones you set up on stage, the greater the chance of one of them producing feedback through your audio system. Run your keyboards and other line instruments through a direct input box, to avoid having to mic their amps. Run lines from the audio output jacks on the back of your guitar and keyboard amplifiers as well.
Turn the volume down to half on all mics that are not being. Do this through the soundboard. Leaving the volume halfway, then restore it to the correct level. This will allow the performer to be heard when periodically returning to the mic throughout their set. Bring the levels back up, once you notice the singer return to their mic.
Minimize the number of times the mics are moved on stage. Find the microphone placement during sound check -- ensuring that it does not produce feedback -- then try to keep them in that general area during the performance.
Reduce the gain levels to below the edge of feedback on the soundboard mixer. Raising the levels to where they are just about to feedback will produce undesirable frequencies, if one of the earlier mentioned factors come into play. Having less overall volume is more desirable than feedback.
Avoid instrument feedback. Guitars produce what is known as string feedback, which is produced when the sound of an amplifier or speaker causes the strings to vibrate. This will produce feedback through your system as well. Turn the volume knobs on guitars that are not being used, all the way down, and have the guitarists periodically mute the strings with their palm, to stop the vibration.