bottlenecker
Poster Extraordinaire
Audio Technica ATM-350, or a Pro35 if you want a cheaper version. I've used mine on nylon string, dreads and small body flattops, archtops, banjo, and double bass. My violin player uses one too. It's a small diaphragm condenser microphone that is twice the microphone an SM57 is for most acoustic instrument use. It's in a miniature form with a gooseneck mount that can clip on to a tailpiece, archtop pickguard, or to the edge of a flattop soundhole to be positioned inside the body. I use it for live and recording.
Any clip on mic is miles better live than trying to play to a cardioid dynamic on a stand.
My best trick for acoustic guitars is I have my acoustic player plug his martin's pickup into his amp on stage for monitoring purposes, because it won't feed back. Then I have the mic clipped into his guitar and going to the mains only, and make sure there is none of that mic in the monitors. Sounds beautiful in the room and is very feedback resistant.
Edit: I should mention I am a sound engineer. I've done the work on this so when I use it at a venue I can tell any sound guy exactly what I need for it to work. Which is, don't put it in the monitor in front of that instrument.
One more tip. It has a high pass filter, or "low cut". Leave it on, even on bass.