First, make sure those tall flowers have support so they don’t flop over or break off. Disbudding—removal of all but the central bud on each stalk—will result in larger more spectacular flowers. Deadheading—cutting the spent flowers back to one node below the bloom before they can set seeds—will encourage lateral blooms. Water the plants regularly, and continue to apply low nitrogen fertilizer throughout the summer.
Fundamentals of Growing Dahlias , American Dahlia Society