There are three different ways to propagate this plant - if the clump is very large, you can dig it up in spring, split it and replant the section/s. Otherwise, you can try semi ripe cuttings, in mid to late summer, from non flowering shoots, some instructions in the link below
http://www.garden.org/regional/report/arch/inmygarden/200
Otherwise, collect the seed when it forms, removing husks and dried bits from the flowers, and sow where they are to flower outdoors, or, more successful, ensure the seed is dry, contain it in some foil and then a plastic bag, or just in a sealed plastic bag, pop it in the fridge to store the seed over winter, and start off indoors the following year 6-8 weeks before you want to plant them out.