1 - Guitar Selection: Every brand and style of guitar has a different sound. Yamaha or Martin, a full size Dreadnaught or a smaller Parlor style. They all sound different, your favorite or most expensive guitar may not be the best for every situation. Having a few choices available will help you get a lot closer to the sound that's right for any song.
2 - Tuning and New strings: It's very simple but often overlooked. Before an important recording session, put new strings on your guitar. Before every take make sure it's perfectly tuned. If you use a capo remember to compensate with your tuning.
3 - Listening: Instead of just putting the mic where you think it will sound good, actually get up and walk around and listen to the tonal changes in each part of the guitar. When you find a favorite spot, put your mic there. This is a great starting point for a mono (single mic) recording as well as a good warm-up for your ears. If the song calls for a stereo acoustic guitar part, you still need to find the sweet spot for the mics. How high or low, close or far, you don't know until you take the time to listen.
4 - Mic Choices and Position: In the studio it is unlikely you will prefer the sound of a dynamic mic on acoustic guitar compared to a condenser, but if you've never heard it, by all means try it, try all your mics. Large diaphragm condensers and small diaphragm condensers are the most common choices for acoustic guitar recording. Again, listen to the differences between mics and where you place them. The closer the mic is to the instrument the more 'Proximity Effect' (an exaggerated low frequency boost in the mic) there will be. Avoid using mics that might exaggerate lows, mids or highs in an already too dark, middy or bright guitar. Pick a mic that complements or balances the sound, dark mic on a too bright guitar for example.
5 - Processing: After you've done your best capturing the guitar right, you still may need to do some work to get it playing nice with all the instruments. In the mix you'll usually need a bit of processing to make room for other instruments, control dynamics, among other things. Generally you need to: cut the very low frequencies, shape the mids to make room for vocals, compress a few dB to even out the performance and add a little reverb to give it space.
