If you have not already done so apply a good quality lawn fertiliser like Neutrog's Sudden Impact for lawns. Then mow regularly about every 5-6 days not cutting too low. Look for any new weeds that have germinated and spray with a suitable weed killer
before they grow too big. In the case of small lawns, they can be dug out using a suitable tool. Do not spray within 4 days before or after mowing.
When mowing keep the blades high so that scalping does not occur. Watch for any signs of
Grass Grub infestation and spray if necessary