Amplifying an acoustic guitar with a microphone can be tricky. The instrument is often susceptible to feedback and sound engineers frequently ask you to use a pickup to compensate. If your pickup system is a soundboard transducer (like piezo elements under the bridge saddles) then the result is a highly unnatural sound that only partially translates the beautiful acoustic sound. However, knowing a few things about microphone placement can help tremendously in taming feedback while still getting a natural sound that only microphones can capture.
In the microphone world, there is something called the proximity effect. The proximity effect describes the tendency for a microphone to pickup more bass frequencies, the closer it’s placed to the sound source. This concept is important in taming low frequency feedback and getting a balanced sound. A common mistake is that guitarists will tend to position themselves so that the microphone is as close to the soundhole of the guitar as possible. The area directly in front of the soundhole is where much of the low frequencies of the instrument are picked up; combined with the proximity effect it creates the perfect situation for feedback to occur. To solve this, try to stay about a foot away from the microphone when possible, and angle it away from the sound hole. The 12th fret is a common target to point a microphone towards. If you’re using a clip-on mic, try and follow these guidelines with the limited mobility of the microphone.
If you’re still getting feedback after placing the mic in the optimal position, try using (if available) a phase switch to reverse the polarity of your signal or use an equalizer to dial out the problem frequencies. Just remember these options are best used AFTER you’ve gotten a balanced sound through microphone placement. I hope this helps in getting a better live sound for you!