Another gear question - miking an acoustic

I rented a P.A. for my band, and recently i've been trying to mike my acoustic. However, when i turn the volume up loud enough so that you can hear it, there's terrible feedback when the mike is close to the soundhole. Then, if i move the guitar away because of the feedback, you can't hear the guitar.

Can anyone help me? The mikes i have are rented too, so maybe it's because they're not meant for guitars?

Also, i'm thinking of getting a pickup for my acoustic. What kinds are good? Soundhole or otherwise? How hard is it to install? Any other suggestions?

Thanks again.

I'm probably not the best person to answer this, but I'm here so I'll give it a shot. As for miking the acoustic, that usually works fine, and some people even like it better. Today me and another guitarist were playing with just a mic in front of the acoustic, and it sounded great. The problems in sound you had probably have something to do with your mixing board or amp settings. Try tinkering. As for pickups, you can get one installed into the side for a small fee (not including the cost of the pickup), and then there are other kinds, for instance one that you just clip onto the soundhole for your performance and then take off after. As for which is better, I think it's mostly personal preference. Do some research on it though.

Feedback is caused when the microphone picks up the amplified, original sound from the speakers and re-amplifies, over and over. You can combat feedback in an acoustic guitar in several ways:
1) Move the guitar and microphone away (to the side or behind) the speakers.
2) Try placing the microphone in a different position on the guitar; try angling the microphone or moving it to different areas around the soundhole.
3) Try a different microphone. Microphones come in several "flavors" - omnidirectional, unidirectional and cardioid (single or dual). An omni picks up sound in all directions, uni picks up sounds in one direction. Cardioid mikes have a heart-shaped pick up pattern. Note that mikes are not really unidirectional - sounds from all directions will affect the mike. However, a unidirectional mike is less sensitive to sounds from the sides or behind.
4) Use an equalizer or feedback eliminator. Notice that the feedback is usually in a certain frequency band. This is due to the natural resonances of the guitar, speaker cabinet, etc. You can decrease the gain of the offending frequencies with an equalizer to help control feedback. Beringer makes feedback eliminators which are narrow-band equalizers that automatically sense feedback and reduce the gain of the band that is feeding back.
5) Reduce the volume of the rest of the band. Sometimes this is the only way to reduce feedback from an acoustic guitar. Hollow-body guitars are more susceptible to feedback due to the way they are constructed. The top of an acoustic guitar resonates with the strings and the body amplifies the vibrations. For this reason, (even) the other instruments will cause vibrations of your acoustic top and complicate the feedback issue.

You have several choices when adding a pickup to an acoustic. You can add a magnetic pickup that fits in the sound hole without modifying your guitar. These pickups work by picking up the string vibrations, like a regular electric guitar, so your acoustic sound may not be projected. Magnetic pickups are usually inexpensive and you may or may not like the sound.
You can also add a piezoelectric pickup that works by amplifying the vibrations of the top of the guitar. Piezo pickups send a very small signal, so you will usually need a preamp to bring the signal up to the levels that your amplifier can use. Some piezos will mount directly to the top with double-sided tape, but most (from Fishman, Martin, etc.) require installation by a guitar tech. Unfortunately, the cost of the piezo pickup, preamp and installation can be expensive. Unless you have an expensive acoustic, it may be cheaper to buy an electro-acoustic that already has the pickups in place. Piezo pickups are very sensitive and can suffer from a "quacky" sound. I believe that this sound is caused when the movement of the top is out-of-phase with the movement of the speakers. Some of the acoustic pickups come with a phase-reversal switch to combat this occurence.
There are a lot of good electro-acoustic guitars available. When I decided to amplify my acoustic, I ran into the same problems that you are having. When I investigated, I found that I would spend several hundred dollars to add a pickup to my old flat-top. I ended up shopping around and buying a Yamaha LD-10E that has both a piezo pickup and an internal microphone. I can blend the two sounds and adjust the equalization to help combat feedback.
Acoustics are always more sensitive to feedback than solid-body guitars. You will never be able to compete with a Les Paul through a cranked Marshall. The secret to controlling feedback is controlling stage volume. Play loud enough to get the sound you want, then mike the amps and instruments and use the PA to bring the audience sound level up.