Wait until the mint forms three or four sets of leaves in the spring to cut them back for harvesting.
Snip or pinch the stems back just above a lower set of leaves for harvesting. Do this as often as you want since mint grows back even fuller than before, provided you care for its other needs. Cut back the younger stems to use fresh and cut the longer and older stems for drying.
Prune back a perennial potted mint in the fall back to the ground. Then, move it to a sheltered location such as a porch for the winter.