Get rid of the dead stuff. While you are at it, get rid of any plant you don't like and compost it. After all, growing the plants you love is one of the secrets to achieving happiness in your garden. Plus you will love your garden if you are at least growing the plants you love.
Clean up your vegetable garden. Not cleaning up your vegetable garden in the fall is like throwing all the Christmas lights in a big box after the holidays and leaving them all tangled up. When you get the lights out of the box again, you are mad at yourself for storing them that way. Don't leave the vegetable garden a tangled mess for spring.
Make up with your houseplants. You know you neglected them all summer. Give them a good shower and a good soaking. Replace that top layer of potting soil with fresh potting soil and trim them up a bit. Put them in the best windows. They will be your new best friends all winter.
Remember fallen leaves are gold. Do not rake them to the curb for the city to pick them up or worse, bag them up for the trash man. Instead, mulch them on the lawn, or rake them up and put them in the compost pile. Crush them up if you can so they'll decompose faster and take less room. Your garden will love you for doing this.
Add a new garden border or bed. Make it just a little bigger, just a little wider. Don't scrimp on size. Plant it now or in the spring. You'll love the results if you do it now.
Evict the weeds. Don't let any of them think they are staying for the winter. Once you think you've evicted them all, go back through and find the "hiders" and get rid of them, too. Your garden will be grateful to be rid of those nutrient sucking, space hogging blemishes of the flower borders.
Mulch your beds and borders. You'll love how your garden looks when the beds are freshly mulched. While you are at it, give each border a nice fresh, sharp edge going into winter.
Envision spring. Try to remember where in the garden you planted bulbs, then go plant more bulbs in other places. If you can't remember where the other bulbs are planted, dig and hope for the best. You will love your garden in the spring if you plant bulbs in the fall.
Leave your pruners inside. Fall is no time to prune. It sends the wrong message to the trees and shrubs. However, if you really must use your pruners, use them to cut back perennials, to make the garden as neat as you'd like. But don't cut back mums. And leave those perennials with interesting seed heads standing so you'll have winter interest to love. Or cut them back if they are over zealous self-sowers that will make you unhappy in the spring when thousands of their seedlings sprout.
Make some notes. Even if you don't regularly write in a garden journal, note varieties of annuals you grew and liked, list perennials you want to get in the spring, and draw a map of your vegetable garden so you can rotate the crops to different spots next year. You'll love having this information in the spring.
Put your tools and hoses away, and the garden decor, too. Winters can be so hard on tools and pots and other garden decor. They'll rust. They'll crack. They'll fade. They'll be crap by spring if they are left out in the winter time.
Finally, leave a door open just a crack every once in a while. It's cold in the winter and the garden fairies will use the crack to get in and spend the winter around the houseplants. They won't be any bother, and some of them will even make toast for you on occasion. (added by Violet Greenpea Maydreams because sometimes Carol forgets.)