The late summer and early autumn are the best times to turn your attention to the lawn. After months of being cut, walked on and perhaps drying out during a hot spell, the average lawn needs help. The good news is that the ground and the air above it are still warm which will help recovery.
Start by mowing the lawn quite short, then go over it with a spring rake or a power-rake, combing out the thatch and moss. Removing the moss before the autumn means that there is less of it to grow and multiply during the cold months. Next comes ground aeration in order to loosen the ground and get more air to the roots (yes, they need to breathe). This is particularly beneficial on clayey soil. A garden fork pushed 4-5 inches down into the ground and wiggled will do it; repeat every 10 inches or so. If your grass is uneven or has bare patches, spread on it a thin layer (8-12 litres per square metre) of proprietary lawn top-dressing, then brush it into the grass and the aeration holes, more over dips, less over bumps. Follow on with an application of an autumn lawn feed and, finally, evenly scatter fresh grass seed at the rate recommended for over-seeding.
September is a good time for establishing new lawns, using either seed or turf. Which method you choose will depend on whether you want an instant lawn or can wait a few weeks before you walk on it. A seeded lawn is much cheaper than a turf one. Whichever way you go, if Mother Nature doesn't provide enough water, you will need to do it for the first 2 or 3 weeks.
Later on, keep autumn leaves off the new lawn, especially if grown from seed.