Last week I had the opportunity to watch an experienced producer do the initial miking to record a traditional rock band in a more-or-less live setting--two guitars, bass, and drums, recorded simultaneously. (Vocals are almost always overdubbed later, even in this kind of set-up. This band was also adding keyboards later.) I've been on the other side many times as a bass player, but this is the first time the producer asked me for my opinions, then explained why he was placing the mikes where he was.
Now, entire books could be (and have been) filled with advice on mike technique, and this producer admitted that there's a ton of opinion and experience involved in the process. But if you're looking for a place to start, here's a quick overview of what I've learned over the years about placement.
Drum kit. The most difficult instrument to mike well, and also the most important, as it usually takes up at least four (and sometimes many more) tracks and is the engine of most rock bands. Some general principles: if the kick drum has a hole, place a large-diaphragm dynamic mike--the Sennheiser 421 is popular--inside the hole, angled slightly toward the beater. If not, place it about a half inch away from the head and slightly off center. For snare, it's hard to beat an SM 57 placed with the diaphragm aligned with the edge of the drum, pointed toward the middle of the head. Toms can be miked the same way as the snare, with the diaphragm aligned with the drum edge and pointed toward the middle of the head; again, 421s are often used here. There's some argument as to whether you need to mike cymbals and hi-hat or not, but most producers I've worked with have voted against it. Instead, they place a couple of overhead mikes directly above each of the main cymbals. Finally, a stereo pair of room mikes helps get the natural reverberation of the room--you don't want the drums to sound too much like they're playing in a tiny box. There are countless opinions on how to set up a stereo pair of room mikes, but the producer I watched last week pointed them away from the kit, toward the corner of the room--he is using a converted church with high ceilings as his studio.
Here's the most interesting tip I learned last week, though. With drums, there will be slight delays between the close mikes, the overheads, and the room mics. You should adjust the timing--easy with today's digital workstations--so the hits are all coming at exactly the same time. But which should be the master? Not the close mikes, but the overheads--that's the drum sound closest to what everybody else in the room is hearing, so if you make it the master, it's more likely to be "tight" with the rest of the band.
Guitar amps. Most engineers use at least two mikes on each guitar amp. One is placed as close as possible to the middle of the cone of the speaker, without touching the mesh over the speaker. If the signal's too loud and it's causing the mike to distort, try and get the guitarist to turn down a notch. (In general, playing slightly softer actually results in a louder recording. That's because the mike diaphragms have more range in which to vibrate...if they're pegged to the max all the time, they actually record less motion.) If that's not possible, simply double the length away from the speaker and the volume will drop 1/4th, or 6dB. The second mike is usually a different to get a different tone, and is placed further away--sometimes near the guitarist's ear to mimic what he's actually hearing. One approach is to use an omnidirectional mike as the second mike--you don't have to worry about pointing it exactly at the center of the amp, as you'd have to do with a cardioid mike, but you won't pick up the rest of the band because omni mikes have very short range. Again, for timing, the farther mike is the master--it's closer to what the rest of the band is hearing.
Bass. Every producer I've ever worked with has recorded the bass with a mike placed near the center of the cone, and with a input coming out of a direct box connected to the bass (not the amp), then mixed them into a single signal. The live mike tends to get more boomy low end, while the direct input gets more of the high-end from plucking the strings. Another paradox: you actually need that treble to hear the bass; a clear attack ensures that it won't sound like washed out mud. I've seen a lot of engineers place the bass cab in a small enclosure to avoid bleed, but the producer last week simply put it on the other side of the room, pointed away from the drums and guitars, separated with baffles, explaining that bass can only sound proper if it's got some space in which to reverberate. Sure enough, there was no bleed, and he got a great sound out of it.
Of course, the most important thing is to experiment with placement and different types of mikes. They're expensive if you're just starting out, but you might be able to beg, borrow, or find amazing deals from Craigslist.