Whenever you use more than one mic on a source you open yourself up to possible phase problems. Make sure you know what an out of phase mic sounds like and how to check for it (hint: put it in mono and listen). That being said multiple mics is an important technique that everyone should master, but know when to use a single mic.
For example, most overheads or room mics are done in pairs--definately check those for phase carefully. But that is just one example of many. I often use two mics (over and under) for snares and 90% of the time I have to put a phase inverter on the under mic... no big deal. However, make sure that your phase inverter doesn't move things out of phase... the cardinal rule is to listen.
Personally I use multiple miking techniques on a LOT of stuff, or at least multiple sources combined with a mic or two. In general I do this:
Kick drum: two mics--one near beater, one almost outside of drum. When mixing I can balance the click to the boom to get what I need for each song. Pretty versatile technique.
Snare: two mics, one under and one over. Same deal as the kick, I will choose the relative volume levels during mixing on a song per song basis depending on how much crack versus how much body I need.
Guitar amps: generally I stick a dynamic (SM7 or SM57) about 6" from the grill off-axis and another mic (cardoid condenser, usually large diaphragm) about 6' away aimed more at the middle of the cabinet... lately an AKG 414BULS when I can get my greasy mitts on one.
Acoustic guitar: I get more elaborate for this because capturing a good acoustic guitar sound isn't easy. You need a good player and a good guitar with a good room, using quality mics in optimal placement to really nail this. At first this is a harrowing experience when you are starting out! I usually point a small diapragm condenser towards the butt end of the guitar and another pointing more towards the 12th-15th fret. I've used SM81's (a bit too bright), C2000's (bright but meaty) and Rode NT1's (a bit thin) in the past. So far I haven't nailed it. If the guitar has a preamp system I will record that too and might use it or I might not, depending on what's what.
Bass guitar: I'll only use one mic on the amp (RE20, MD421 or AKG D112... MD421 is my current favorite) and mix it with a direct signal. I've DI'd it straight before but am warming up to devices like
the Sansamp RBI. For mixing this the direct signal is usually the low end and the amp will provide most of the tone.
Piano: I haven't mic'd a lot of pianos for recording (because I'm a synth geek and have digital and analog synths all over the place) but I have done it for some live performances I did sound for locally. You definately want at least two mics, and I recommend using a small diaphragm condenser.
