Garbage In, Garbage Out...
If what goes into the system sounds rubbish, it'll come out rubbish too. To that end, let's look a little closer at miking up our instruments and vocals...
The mic should point at the mouth (duh...) either directly in front of the mouth or just below, at the chin. Just below is better as singers often look down at their music. The mic should be three fingers away from their mouth.
Get the mic close to the strings, positioned over and pointing at the bit after the bridge where the strings connect to the body. Simple
Electric Guitar Amps
OK, so there's loads of options here, but basically, stick the mic between the centre of the speaker and the edge and close to - but not touching - the grill. That sould give you a balanced sound.
With our mics, aim to have the mic pointing directly away from the foldback speaker. (But be aware that this isn't true for every mic, it's just that we only have cardioid vocal and amp mics) This will reduce the chance of the mic hearing itself and producing feedback.
For much more info: Mic Guide.