Fall is a time to care for your lawn and prepare it for the harshness of winter. You are going to strengthen your lawn, so that when spring peaks its head out of the cold clouds, your lawn will be ready to remerge, vibrant and ready for the spring time festivities.
First, you should lower the blade on your mower, and take your lawn down to no shorter than 2-2 ½ inches. You want to cut the grass back to strengthen its root system and crown. If you are in a damp or snowy region, this will also help to limit the risk of fungal disease. This disease will show itself in spring, and you'll find dead brown portions of your lawn.
Second, aeration should be done in September or October. Aerating opens up the hard compact soils. This will allow the lawn to more readily take in nutrients, air, and water directly into their root system.
There are a few options for aeration. One machine simply pokes holes into the lawn, pressing parts of the lawn and soil into a hole. The other aerator punches plugs into the lawn, pulls them out, and leaves them on top of the ground. Although it is a little more unsightly, it gives a better opening into the grass for nutrients to be placed, and the unsightly plugs dissolve with time. If you are not able to spend money on an aerator, the use of a pitchfork can be employed. Simply throw the fork into the ground and wiggle it around, allowing small holes to be widened by the movement.
Third, fertilizing is key to fortifying your lawn. It is best done after aerating. This will allow your lawn to save needed nutrients for the long winter. Make sure to have this process completed at least one month before the first frost.
You want to use a fertilizer that is high in potassium and phosphorous. It is also a good time use some lime in your mix, which will reduce stored acidity in the soil. Your lawn should get about 1 inch of water per week during the fall, so supplement with a hose, if your area is running dry.