I cut mine WAY back after bloom with garden shears. This keeps it nice an compact and it looks great. Sometimes I get a second bloom. Also, if you shear once BEFORE it blooms the first time, you'll get a more compact plant in in general.

The Well Tended Perennial Garden by Tracy DiSabato-Aust is an awesome book for perennial management. She's a big fan of pruning and shearing to control bloom, growth habit, sprawling and general looks of plants.