There's good advice from Zemlin in his reply.Originally Posted by maneli
I would position the mic not too close to the instrument, otherwise the guitar will smear itself across the entire width of your L-R sound field. Try starting at about 2ft (600mm) away from the instrument, perpendicular to the point where the fingerboard meets the body. To maximize the on-axis sound, the X-Y plane of the microphone should be aligned to how the player holds the instrument, i.e. if he tilts it up at 25 degrees, then twist the mic by that amount away from the horizontal.
It's more difficult to find a good position acoustically using your ears alone when you are searching for a position for a stereo mic, so high-isolation headphones being fed with the full stereo image is a good way of working. Experiment by moving the mic around while listening to the captured sound.
You will need to instruct the player to keep his body and playing position still during the performance - particularly important when using stereo mics on acoustic guitars to avoid the image wandering.