Educating users in mic technique can also help. Many people tend to grab mics or lean very close when they speak. Both of these actions can cause problems. Grabbing a cardioid microphone can increase its physical gain at certain frequencies when the user's hand closes off the rear ports to the microphone element, making a stable system suddenly unstable. When people lean too close to a mic, they can reflect some energy at problem frequencies back into the mic, possibly causing feedback.
Some reverberation units can cause an otherwise stable system to become unstable.
With a simple enough system, polarity or phase changes could solve a feedback problem immediately. However, in large, complex systems with multiple return paths many wavelengths long, phase or polarity changes might just shift the feedback frequency, without increasing stability. The most common solution to the problem of feedback is to use equalization to take out offending peaks. Remember that you are reducing gain, even though you are reducing it only at specific frequency system to increase stability. In many cases, the frequencies in question have proportionately too much gain anyway.