Dig up the root ball of the plant with a hand spade. Start digging at the widest point of the plant's foliage, and dig under the root ball so you get all of it out of the ground. Do this in the spring, after the last frost.
Pull the root ball in half with your hands. If the roots are tangled and will not come apart, cut the ball in half with the sharp edge of the spade.
Replant one half in the existing hole, or plant both halves in new locations. Repeat the division every three to four years, or whenever the plant starts to fall open because of age.