Get a soil test. Assessing your soil health now gives you time to correct nutrient deficiencies and pH problems before spring.
Shred the leaves. Fallen leaves can smother a lawn if left in place all winter. Use a mulching mower to shred leaves into vital organic matter that you can leave right on your lawn.
Throw off thatch. Compacted soil and too much thatch—an accumulation of undecayed and decaying plant matter at the soil surface—denies grass roots the air, water, and nutrients they need to thrive. (Thatching, by the way, is caused by excess fertilizing, not by mulching grass clippings.) Increasing organic matter will stimulate the soil microbes that consume thatch. If the problem is so bad that water cannot penetrate the thatch, remove the thatch now with a stiff rake or thatching rake.
Mow high. Continue to mow grass until it stops actively growing. For the final mowing of the season, cut cool-season grasses to 2½ inches and warm-season grasses between 1½ and 2 inches, which is just a little shorter than you should cut it during the spring and early autumn.
No fast food. Fertilize cool-season grasses in fall with a slow-release, organic fertilizer, such as Fall Lawns Alive!® Application timing varies among regions, so check with your county extension office for local recommendations. Don't fertilize warm-season grasses in fall.
Reseed bare areas. Prevent weedy patches next spring by seeding now. Grass seed grows well in fall because the temperatures are perfect for cool-season grass and because it has less competition from annual weeds. Just be sure to give the lawn enough time to establish itself before winter weather hits. Plant and renovate warm-season grasses in the spring.
Keep Reading: The Dark Side of Lawns.