From my experience of sound engineering live PA for acoustic instruments.
1. If the feed back is high pitched it is the monitors. 2. low pitched usually means the front of house speakers. 3. Electric guitarists who use feedback (Carlos Santana) used to due a soundcheck to FIND the spots that caused feed back best and marked them wit tape! You can do the converse here walk around with your guitar with vol turned up and find best/worst spots - good sound man will help here. 4. use a directional mic (cardiod - heart pattern) they tend to be long a thin. The ones with the large round bulbous ends tend to me omni (multi) directional and will pick up sound from behind - ie the monitor sound (with YOUR guitar sound) in them. 5. Most acoustic type feed back accurs at approx 300hz 6. A 31 band equaliser will help (around 300hz) but if you are using a sound person they should be doing this for you. assuming that you are BEHIND the front of house speakers this should be put into the MONITOR circuit, this will cut the offending frequency FROM THE MONITOR and hence cure the feedback but WILL NOT take the frequency from what the audience will hear on the front of house. 7. Feedback never just HAPPENS it builds up ie it feeds on the original signal, amplifies it, transmits it and so on. As a temporary measure as yopu hear/feel the soundboard of the guitar vibrating just press your forearm on the soundboard this will deaden the buildup. 8. BERINGHER have a device called the DSP-1100 Feedback Eliminator cost approx 110 uk sterling. This can be used in a programmed mode - ie you play your insrument into it and when it detects feedback it will down the frequency a knotch and you try again until it is at an acceptable level. It also works in an automatic mode whereby in a live performance it detects feedback automatically and then dampens that frequency to eliminate the feedback. This is a more agressive damping than the pre-programmed type but it stops almost instantly the Wummph or screech of the feedback. One very important difference between this unit and a 31 band aka 1/3 octave graphic equaliser is that it ONLY attenuates the ACTUAL FREQUENCY causing the trouble ie 313 hz and not 280-330 for example. Very clever bit of kit. Can be used on front of house and monitors from either direct instruments or the two outputs from the mixing desk (ie ALL the bands equipment and vocals) highly reccomended. Bev Lawton