Classic metal cocktail shakers with threaded tops can be the bane of a bartender's existence. They look nice, but have an inconvenient habit of becoming stuck together. This presents a problem because with the two halves conjoined, you obviously can't make any more drinks.
Empty the shaker of its liquid contents.
Wrap the dish towel securely around the shaker.
Tap the shaker against the bar, at the connecting point of the two halves, rotating to strike it evenly all the way around the ring of the shaker.
Twist the two halves apart.