Deadheads need to be removed on a regular basis. This is particularly important in the case of bedding roses and rose bushes. The deadheads should be cut off along with the two leaves lying underneath them. Cut back delphiniums and other herbaceous perennials once they stop flowering. They will then usually flower a second time.
Dahlias start to flower from August onwards depending on the variety. Cut large sprays to encourage new shoots as well as giving you and your neighbours, perhaps, a little extra pleasure.