Recording acoustic guitars (not electro-acoustics, just the plain old wooden box with strings on) is an art all it's own. Ideally you
want a good condenser microphone for the job - though you can make a
passable effort with an ordinary (dynamic) mic.
As with all recording projects, the ideal situation is to get the best sound you can at the source, rather than trying to fix it later with
EQ and compression (though there will always be a necessity
for small adjustments post-recording), so you should take plenty of time with
headphones adjusting the position of the microphone to find a "sweet spot".
Generally speaking, I find that about 9 inches to a foot from the guitar
gives a nice signal (you may have to adjust this depending on the style and
power of the guitarist you're recording, a little nearer for fingerpicking styles, a little further for busker style bashing)
and then pointing more towards the sound hole of the guitar gives a bassier,
fatter sound. Pointing the mic more towards the neck of the guitar gives a
sharper edge of string definition. Adjust the mic position until you hear the
sound you want.
Then hit record and get it recorded before the guitarist accidentally kicks the mic stand and you have to start all over again.
It's worth noting that recording and mixing is a highly subjective art - these techniques may simply not give you the sound you want to hear, if so you can't beat the old tried and tested studio formula of not having a tried and tested formula. The best hard and fast rule I can give you is: If it sounds good, it probably IS good.