I do not have a pickup in my acoustic but when I went inquiring about the possibility at the local guitar store I got some advice.
The dude said the best thing he had in stock was this bar magnet pickup. There are ones that go in the soundhole and more expensive ones that you attach to the body of the guitar inside the soundhole.
In any case, go for a piezoelectric (crystal) pickup or a bar magnet type... soundhole mics tend to sound pretty bad...
Hopefully someone else has more knowledge on this subject. This is essentially a bump.
edit: just found this... kind of an outline
CHOOSING AN ACOUSTIC TRANSDUCER
If you already own an acoustic guitar or are thinking about buying one an acoustic transducer is the perfect way to mike it up for stage or studio use.
Many modern acoustic guitars come equipped with built in electronics but ask yourself this - how often does electronic equipment get updated? Every 6 or 12 months? But what about that new Taylor that you saved for, for what seems like forever, which was meant to last you for years. If you buy a pure acoustic guitar you will be able to update the pickup or transducer as new technology becomes available without any damage to your precious guitar. That is very difficult if not impossible to do with factory installed pickups.
Acoustic transducers fall into three categories:
The easiest pickup to fit to an acoustic guitar and the least expensive is a magnetic pickup, which is basically an electric guitar pickup optimised for acoustic use. However, magnetic pickups tend to be the least realistic in terms of reproducing acoustic guitar tone.
Piezo pickups either attach to the sound-board or are mounted under the bridge-piece. they work by converting sounboard vibrations into electrical impulses. While they give a more accurate sound than magnetic pickups, they are more expensive and can be more difficult to install, typically needing a jack-socket to be installed using an end-pin jack-socket.
Hybrid systems like the Fishman Rare Earth combine magnetic pickups and internal microphones to give a more accurate rendition of the guitar's tone. However, they tend to be the most expensive and require an end-pin jack socket also.