1. Know the frequencies. In order to eliminate feedback you must be able to identify the frequency feeding back.
2. Use proper speaker placement. Keep the mic line upstage of the PA Speakers.
3. Ring out the vocal microphones in the PA.
4. Start with good sounds at the source.
5. Use the right microphone. Pick the right type of mic and pickup pattern for your needs.
6. Use proper mic placement. Close mic’ing is the preferred method in live sound.
7. Understand and implement proper gain structure of your inputs.
8. Use subtractive EQ rather than additive.
9.Understand signal flow to avoid unwanted mechanical feedback.
For a more in depth explanation on any of these download the FREE ebook ‘7 Things Every Live Sound Engineer Should Know’ .