Get ruthless! Any summer crops that aren't producing need to make way for the fall crops. Tomatoes and peppers that are full of flowers look hopeful but will probably not bear fruit by frost, so cut your losses, pull them out, and plant some fall cole crops or greens. I'm very happy to see that the big-box stores are carrying transplants, something they've started because of popular demand.
Think about cover crops. These are things you plant now in the empty spaces left by the slacker tomato plants; they will enrich the soil over the winter and keep the ground from being naked. Planting some sort of pea family cover actually pulls nitrogen out of the air and holds it in the soil for us to take advantage of later. Even better, any cover crop you plant also takes up carbon and sequesters it in the plant scraps, so the earlier you get them in, the more plant residue they leave in the spring. The secret is not to over-till in the spring, because tilling rereleases that bound carbon into the atmosphere. So, more plant matter + less tillage = less CO2 in the atmosphere. Words to live by. Adam from City Harvest likes an oat and pea mix, as cold winter temperatures eventually kill off the plants, and you can just plant through the residue in the spring without turning it in.
Keep fruit trees scrupulously clean underneath, because fallen apples and pears attract yellow jackets. It's especially fun to watch them as they are consuming fermented fruit: A little alcohol goes a long way in such a small creature. But watch from a distance. We know how cranky they are this time of year.
Take advantage of fall sales. There is so much color available right now to jazz up your landscape. Look for fall-blooming perennials, and shrubs that promise brilliant reds and oranges. I like to place them, still in the pots, where their brightness makes the biggest impact, and move them around until they're just where I want them before I plant them permanently. Just don't forget to water them every day in the pots until they're in the ground.