Landrover, cut off the spent/rained off flower heads back to the nearest branch or stem. They should continue to produce buds/blooms for you "all summer" like the ads say. You don't want to let a plant go to seed though, or it will think it's done it's reproducing job and be done.

If you keep cutting flowers (or shearing off the old blooms) it will have to bud some more to try to keep itself or it's offspring on Earth.