One trick is to put your head right in front of the amp and find it's "sweet spot", the spot that sounds best to your ear, then place the microphone there.
Types of microphone for this purpose are either dynamic or condensor and more can be read about the differences here.
If you're recording with one microphone per amplifier the different sound sources can bleed together; the microphones loose the definition of what they're supposed to be recording. To help counter-act this problem you can put a can/cup holder (usually for keeping drinks cold) around the microphone so that it's only receiving sounds that are coming from the amplifier directly infront of it.