Guitar amps that are out of phase can create a nasty voltage between
them if you are using a multi-amp setup. (Read: you'll get a painful
shock if you touch both amps at the same time.) The way around this is
to keep the chassis of both amps grounded together. If the chassis are
grounded together, you'll blow a fuse before possibly electrocuting
The SM-57 is probably the most popular mic for a guitar cabinet, although many mics work great.
In a multiple speaker cabinet (2 x 10, 2 x 12, 4 x 12, etc) check all the speakers because one may sound better than the others. This takes a little patience until you know the general sweet spot. Keep the mic angled at a small angle to the grille cloth (not perpendicular), and about 1" out. Point the mic at a portion of the cone, not near the center or edge. Crank the amp as loud as possible to get the sweet tone. Make sure you turn your gain down on your mic pre, as this tends to be a hot signal with a loud amp.
If you invest the time to fine tune amp settings, guitar tone control settings and mic placement to get the best recorded sound, it will pay off handsomely in the final mix. When tracking, take a preamp output and record it to a second track, because the high bass and mids are much more defined. You can then combine the two tracks for a fuller sound.
Here's a tip for recording guitar that works well if you're recording on analog tape. Try running the levels to tape a little hot (+3 to +6dB). A small amount of distortion from the tape machine can sound good sometimes. Experiment with this by splitting your mic signal into two channels. Record a miced amp on both tracks. Record one track at 0 dB peaks and the other with 6 dB peaks. On playback, match the volume of the two channels. When you listen to it, you should be able hear the distortion from the tape. Note: This works for analog recorders ONLY; a digital deck will clip very ungracefully, and should never be pushed over 0 dB.
If you want a little natural reverb, try micing the amp in a mid to
large sized room and use a distant mic as well as the one in front of
the grille cloth. You may have to play with the location a bit to find a
good one. If you hear certain frequencies drop out and acting
strangely, check for phase problems. You'll need to change the distance
between the source and the far mic to remedy it. The caveat here is that
this technique doesn't work well in rooms with poor acoustic properties.
You'll also need quite a bit of gain on the distant mic, so a quiet
place to record is important. It also helps if you use a high output,
low self noise mic. This is a good technique to use on acoustic guitars
as well. Using a multiple amp setup is a very nice way to get a full
sound on the electric guitars. Y-cords can be used to split your signal
to several amps, all miced individually to get a diverse, thick tone. If
they are tracked separately, you can also add a little time change with
a delay and really thicken things up.
Experiment with using a clean amp and a distorted amp at the same time. The multiple amps can be in different rooms, or together and recorded as a whole. In any set up like this, you must pay attention to the phase of the signals. Sum all of the signals to a mono buss, and listen to make sure the sounds are working together. You may need to reverse the polarity of one or more mic, or relocate the mics slightly to correct the problem.
If you decide that you want to try recording an electric guitar direct into the board, one method is to use a preamp output from the back of your amp. It is important in this scenario to utilize a DI to match impedances and eliminate hums. Note that you absolutely must leave a speaker load attached to your amp. This is especially important with a tube amp. You'll notice that this type of signal generally sounds harsh and edgy (even more with solid state amps). This happens because a lot of the characteristic sound of a guitar amp comes from the power amp and speakers. This is why most engineers prefer to mic a guitar amp as opposed to running the guitar direct.