Hey Little Z,
I feel your pain.

Nothing sounds quite as dead as an
acoustic guitar that doesn't sound full. I wondered for a long time why a live acoustic could just fill the room with sound but sound like a toy as soon as I put a mic in front of it.
I discovered a couple secrets, though, and my recordings have never been the same since. Even with inexpensive mics you can get a *killer* sound by using stereo.
Ideally, I'd recommend that you buy another sm57 and use those two mics in an x/y pattern. Pan one hard left and the other hard right and listen to that guitar just jump out of the speakers.
If you're not in the market for a second mic at the moment, try laying down the guitar twice and pan those right and left. It won't sound the same, but it will sound much fuller than one guitar in mono.
The way I see it, the acoustic really does fill the room with sound. When you use just one mic to capture all that sound something is sure to get lost.
I know its tough getting 2 tracks of anything with a 4-track, but that's the best tip I have for getting a good acoustic sound.
As far as mic placement- I put my mics right in front of the soundhole about 10 inches out. It always sounds boomy there, so I angle myself back and forth by swiveling on my stool until I get a sound I like in the cans. Sometimes its with the mic pointed around the 12th fret, sometimes pointed more towards the body and bridge. If that doesn't work, I try moving the mic closer or further away.
Good luck,
Chris