There are a lot of great perennials that will rebloom with a little maintenance. Here is a list:
Aquilegia spp. - Columbine: Cut off blooms on the side stems but always keep a few seed heads for self seeding.
Dicentra eximia - Fernleaf Bleeding Heart: Cut the stems of the flowers all the way down to the basal growth of foliage.
Platycodon grandiflorus - Balloon Flower: Remove spent flower heads leaving some to reseed.
Gaillardia x grandiflora: Snip off flowers down to node for rebloom.
Delphinium elatum: After first blooms are done, cut back to basal growth to encourage new blossoms.
Polemonium caeruleum - Jacob's Ladder: After blooming, cut back plant entirely. After second bloom, leave some seedheads on for reseeding, which is the answer to keep this short lived perennial going!
Campanula persicifolia - Peachleaf Bellflower: Snip or pinch individual flowers but be aware of the developing buds that can be easily removed if not careful.
Salvia nemerosa - Salvia: Snip flower spikes down to side branches or nodes. If completely done with first set of blossoms, cut down to basal foliage for a possible rebloom.
Coreopsis grandiflora - Threadleaf Coreopsis: Deadheading each individual spent bloom is time consuming, but will keep it blooming almost all summer long. For easier rebloom, shear back after first flush of flowers.