Placement of the mic to the front of the soundhole and the top of the fretboard can also eliminate feedback. Use a unidirectional mic (preferably a condenser mic with a flat response i,e all frequencies are reproduced the same way they sound) Then there are some guitars that have a resonating frequency (like singing in the bathroom) where there is a "boom". This is when you need to have a 1/3 octave equilizer (aka a 31-band EQ) so that you can kill that particular frequency without affecting the rest of the sound,