all the above suggestions are excellent.

assuming you have a SM58 and a condenser microphones: it depends if you want a tougher bluesy sound or the natural sound of some one playing on the porch. seeing that you're here at Garage Punk you'd probably want something more raw and rough. i prefer to play the harmonica through a handheld microphone (SM58 or the suggestions above) that is plugged into a guitar amplifier, with reverb. adding some type of delay/tape echo effect pedal really spices up the sound to get a honking authentic 50's sound. i prefer using smaller amps as they don't tend to feedback quickly. use your condenser mic to record the amp. depending on where you set up the condenser mic you can find a sweet spot combing the raw ambient sound of the harmonica player mixed with the amplified sound. it can take a while to find the spot but it has a great effect for just two mics, an amp and a harp.

lemme know if that helps...

Harmonica mic. 

Crystal element. Most people use a shure green bullet type thing. 

I use a Crystal element Shaker Retro Rocket. 

If I didn't I might use an AKG 414 and compress and eq it to radio bastard. 

Harmonica sound awful anyway.