Flowers: Cut back and deadhead
-- Petunias getting ratty-looking? It's a good idea to cut all your annuals back by one- to two-thirds to encourage fuller, bushier growth with more flowers.
In Zones 3-5, if your cool-season annuals such as snapdragons and lobelia have made it through summer's heat (they often die out in July and August), cut them back by about half to encourage a fresh flush of growth and bloom in the cooler temperatures of fall.
-- Deadhead flowers regularly. Deadheading not only keeps your landscape tidy-looking, it also encourages certain flowers to bloom more.
-- Tackle diseased plant areas. If the plant is an annual and badly damaged, consider just tearing it out. This time of year, it's not likely to recover and will only look bad and possibly spread the problem to its neighbors.
Otherwise, trim off an affected part of the plant and take it to a quality local garden center. They'll be able to diagnose the problem and recommend a solution.
-- In Zones 4-6, stop fertilizing roses and any other trees and shrubs. Fertilization will encourage new, tender growth that will get zapped by winter cold.