Jot down notes while the season is fresh in your memory about what did and didn't work, insect and disease problems, and what plants you need to add/move or divide. Refer back next spring to avoid similar problems, or repeat what worked well.
Pull out old summer annuals and plant pansies and spring blooming bulbs. Keep pansies deadheaded for extended bloom. Apply repellents if deer, squirrels or rabbits are a problem.
Clean up any scattered bulb skins and apply mulch over the disturbed soil to keep squirrels from discovering newly planted bulbs. If your squirrels are particularly hungry, lay a piece of chicken wire or hardware cloth over newly planted bulbs, weighed down by bricks or stones. Remove before bulbs sprout in spring.
Lift tender plants you'd like to save for next season like dahlias, elephant ears, and zonal geraniums.
Cut back perennials as needed. It's also a good time to transplant most perennials.
Water evergreens well (both needled and broadleaf) if soil seems dry going into colder weather. Inadequate water supply can lead to winter-burn of leaves and needles during cold dry weather.
Keep leaves raked or blown off of newly seeded lawns so tender grass seedlings are not smothered.
Add "free fertilizer" to an established lawn by finely mulching fallen leaves with your lawn mower and leaving on the lawn. (Not too thick-you still want to see the grass easily. If you have a lot of leaves you will probably still need to rake some.)
Rake up and discard rose leaves in the trash if you've had any problems with rose leaf diseases this summer.
Take advantage of a warm, sunny day to spray perennial weeds with herbicide.
Clean and sharpen your gardening equipment before storage.
Empty birdbaths and fountains and turn over, store or cover them to avoid cracking. Store fountain pumps where they won't freeze. Cover water features with netting to keep leaves out and fish safe from hungry birds.
Mulch beds to reduce frost heaving, protect roots, and add organic matter to the soil.