Rarely does a guitar amp sound good dry, with no reverb or room sound on it (although sometimes it does). The same applies for acoustic guitars'"they need to breathe sonically to get the right sound across. Lets take a look at 5 quick tips using ambience and reverb to help you sit those guitars just right in the mix.
Adding a room mic when recording electric guitars can help add depth to the sound. Try setting up your mic at least 3 feet from the cabinet, and preferably more. Listen to your room and pick a sweet spot, then place as good a mic there as you can. I prefer good omni mics or large diaphragm condensers, as they will deliver the biggest tone. Try not to use something like an SM57, as it tends to accentuate mids. I tend to point the mic right at the speaker, up at least three or four feet high. But sometimes pointing them into a corner can be very cool. Experiment with your ears.
If you don't record with a room mic, or are using a guitar amp plug-in, try adding a room simulator if you want some depth. That can be as simple as a basic reverb with a room preset (most of them will have this), or something like Audio Ease's Altiverb, with real room sounds (recorded impulse responses of the actual rooms). By sending your guitar to a room effect, you will simulate the sound of using the above mentioned room mic. In fact, it may even offer you more control than an actual room mic'"since you can tweak the settings.
It's often discussed whether or not to record your amp with the reverb on or off. While that is an individual decision based upon each production, the general rule of thumb is not to make it too wet'"if you do use it. Remember, that reverb on the amp may sound good, so you may crank it, but you can't take it off in the mix stage. So err on the side of caution, and maybe turn it down a bit. You can always add more later if need be.
Acoustic guitars vibrate and the sound from the body and top needs to project. With that in mind, most people mic the soundhole by the neck. That's great, but try adding a mic on the body for some extra bass response. I'll sometimes add a room mic on the acoustic, about three feet out , facing down at the guitar from a height of about four or five feet. Again, try and use the best mic you can (not necessarily that old SM57), but more like one that will reproduce the guitar without coloration. It can really make an acoustic track come to life.
With electrics, don't just settle for the vanilla sound of the room mic. Try adding some aggressive compression, with something like an 1176. By doing so, you're upping the excitement level of the track and often bringing out the sound of the room even more with the compression. Try panning it to the opposite side of your amps panning, to make a big wide soundstage.
Learn more about producing and mixing guitars in this video course by Rich Tozzoli: