Chasing the threads with a tap (or Larry's slotted bolt) is good if the rust is really pervasive. Otherwise, I have found that a small caliber brass cleaning brush and solvent works well. Just chuck it in a small drill and run it in and out a few times. Don't try to reverse the brush to bring it back out as the bristles don't usually last long being bent back and forth.

just my $.02,

Wes