In my audio travels I hear it uttered all the time: "I can never record a good acoustic guitar sound." Well, maybe after you read this you'll think a bit differently. There are several methods for getting a good sound down, and plenty of acoustics out there on CD, so you know it can be done. Following are a few tried and true methods I've used with excellent results.
First, some basics -- they may be obvious, but they need to be stated. Though your recording can be enhanced with EQ and effects, your fundamental building block is the guitar itself. This is where a lot of problems start. $200 guitars usually yield $200 results. Sometimes guitars like these can be used to fill out a sound, but your best bet is to start with a great sounding acoustic. For session work, I often borrow guitars that I know have the killer sound.
Next, choose a good environment, such as a wooden room with nice natural reverb. Remember that carpeted rooms will usually provide a relatively flat or dead sound. Ambience is very important to a good recording. Walk around the room while playing the guitar and try to find a sweet spot. You might get a nice sound playing into a corner, for instance, where you have the benefit of sound reflecting off the walls. Experiment.
If you do use the guitar's internal pickup, use it sparingly and in conjunction with microphone. Direct pickups usually have a very "honky" midrange sound but can add some high-end sparkle to the sound if used well. Typically, acoustic pickups are designed for and most effective for live applications. When it comes to recording, you'll want to use the best microphones you can afford, high quality cables, and a good mic preamp. Rent them, if need be. These are key components to recording a good sound.
Working with Al DiMeola on his Telarc Records release Winter Nights, I used the "X" method with a pair of spaced omnis. The X technique simply crosses the tips of two cardioid pattern microphones to capture the sound coming off of both the bridge and neck of the guitar. This is a stereo method, and you should pan the two mics hard left and right on your console to achieve the proper "width." For Al, I set the mics up about six inches from the soundhole, with one pointed at the neck and the other crossed over it, pointed at the bridge. Again, experiment by moving the mics closer and further, as well as to the left and right of the soundhole. In case you're interested, the gear I used here included Schoeps CMC-5 mics his 1949 D-0018 Martin, Monster Cable XLR cables, and a Demeter tube preamp.
To further enhance the sound of Al's guitar, I used two Earthworks TC-30K omni mics, spread out about three feet apart and set back about two feet from the instrument. This captures a wide array of the guitars frequencies, and also includes the ambient room tone.
Like before, pan these hard left and right, and mix the channels with the X-pattern cardioid channels. As you bring up the omnis on the console, you will notice the character of the sound widen, with more of the strings' high end and increased room tone becoming present. Achieving the right blend is key to the final sound. You'll usually want the cardioid X channels mixed higher in volume than the omnis, though with good sounds in the mics you'll have plenty of possibilities. As an overall rule, I rarely compress the sound, and rarely EQ. Spend the time on mic setup and get it right to start rather than planning to fix it in the mix.
On one song from Winter Nights, I used an additional two GrooveTube AM-62 tube mics (set back about six feet, and six feet from each other, in a tiled room) for a total of six microphones. The guitar, a beautiful Conde Hermanos classical, was tuned to low C, capturing a huge, rich tone. Again, starting with a great guitar is very important.
I also recorded guitarist Ricky Byrd (Joan Jett, Roger Daltrey) playing a Guild J-50. I used a single Earthworks omni pointed towards the the soundhole, near the top of the neck. A single omni is usually better than a single cardioid because the pattern records a broader band of the guitar's frequency spectrum. In a situation like this, the acoustic guitar's pickup could have been recorded and blended into the sound of the mic for some additional high end. But alone the omni yields an excellent natural guitar sound, and conserves record channels.
One final note: For those who sing and play at the same time, use the bleed of the vocal mic with the guitar mic to create natural reverb. Don't worry so much about isolating channels -- natural reverb can be a good thing, so use it! Natural effects like reverb and the ambience captured from microphones will lend your recording a real-world sound. After all, in most cases that's what we're trying to do; fool the ear into thinking it hears live instruments rather than a recording.
Achieving a good acoustic guitar sound is not hard. Try some of the examples given, and experiment with panning and room sounds. Borrow that pricey axe if need be, but get a great sounding instrument before you begin. You'll be proud of the results from your extra effort.