Dead head those flowers!
People have asked me how I keep my zinnias nice looking into the fall. The answer is “I cut them”. Many plants will go dormant and start looking ragged once the flower head starts going to seed. If you are quick to pick off spent blooms the plant will send up bright new ones. This is true for Zinnias, bachelor buttons, Dahlias, some roses, and others. This is also true with many bedding plants.