Fall is the perfect time for some garden cleanup. Many plants will have passed their peak and be ready for tidying, the temps are often more comfortable to work in and minimizing plant debris helps with disease prevention for next year's garden. It also allows you to more easily see which plants may need dividing, pruning or replacement once the general cleanup is done.
Begin your cleanup by pulling or digging-out summer annuals, which can be placed into your compost pile or bagged with other yard & garden waste for pickup. If any were showing signs of disease, it is best not to put them into your compost. Perennials can be cut down to within 2 to 3" above soil level and the cuttings disposed of as with the annuals. Late season perennials may still be going strong and can be left until finished to be cutback. In cooler zones, where the garden doesn't grow year round, you may wish to leave some "finished" perennials such as ornamental grasses or Echinacea for winter interest in the landscape. Just be sure to cut them back in early spring to allow their new growth the best start without being shaded by, or smothered under, the previous year's growth.
Be sure that any plants that are not quite hardy in your zone, but you want to keep, are protected. Depending on the plant, this may mean bringing it indoors as a houseplant, moving it into the garage or basement until spring, or simply providing it with protection from cold and wind where it is planted.
Mulching after cleanup can be a beneficial finishing touch. In cool zone gardens, especially those that don't get much insulating snow cover, a layer of mulch in autumn can provide protection from the potential of damage due to temperature fluctuations. Adding mulch in any zone will help with water retention and soil enrichment.