Mowing the lawn
Continue to mow the lawn, but reduce the frequency. From mid-onwards it will also pay to raise the lawnmower blades a notch or two. 

Lawn Care
Rake the lawn to remove all the lawn clippings which have accumulated during the growing season. The rubbish collected can be put on the compost heap, or burnt if contains creeping weeds. If the raking is closely followed by a lawn mowing, the blades will chop off any of the raised weeds which remain.

Spike the lawn with a fork, 3 to 4ins deep at 6 to 8ins intervals. this will aerate the lawn and improve drainage. 

Finally, fertilise the lawn with a slow-release fertiliser such as bone meal. Do not use a general fertiliser which is rich in Nitrogen. The fertiliser can be applied by hand, scattering a handful every couple of square yards.