Remove grass from low areas in the lawn. Use a shovel to peel off the grass layer and set it aside. Fill the low area with soil to remove the dip or hole. Tamp it down. Make sure not to build the area up level with the rest of the soil height not the grass height. Place the grass back on top and water it. Keep it watered daily for the first few weeks until the roots take hold again.
Treat weeds with a selective weed killer. Selective herbicides are designed to kill certain vegetation. Spread or spray as directed. You can use pre-emergent herbicides to kill annual weeds by applying them a few weeks before they typically appear. As an example, crabgrass pops out in early summer, so spreading a pre-emergent in early May to mid-May will help kill this intruder. Post-emergent herbicides work on weeds that have already grown. Liquid applications work best for this kind of herbicide because the chemical needs to be absorbed through the leaves.
Reseed lawns where you have killed a lot of weeds or have had to remove dead areas. Reseed using about half the quantity of seed used to start a lawn. The brand of seed will have the exact instructions for the amount to use. Spread seed in a crisscross direction. Water the seeded area and then spread lightly with straw. Water daily for the first month to keep soil damp and then water as needed--twice a week for 15 minutes, for example.
Spray fungicide to kill mold or mildew buildup on the lawn's soil. Water in the early morning to prevent recurrence. Watering at night allows water to sit and provides the perfect growing conditions for mold.
Water less than you typically do to encourage grubs to live elsewhere. Any amount of reduction will help, however, your goal is not to kill your lawn. Try watering only once a week instead of twice or cutting the time in half. If grubs are an issue in your neighborhood, having the greenest, well-watered lawn could be your downfall because it will attract beetles to lay their eggs in your yard. The newborn grubs will then make a meal of your grass roots. Watering less often will discourage this. If you don't want to water less, you will have to spray a pesticide or spread Milky Spore on the lawn. You can also spray nematodes, which will eat the grubs, or mow at a higher length.
Remove thatch buildup and promote growth by raking out the dead grass.
Test the soil if your grass is growing but doesn't seem healthy. The pH might be off. Most grass does best at a pH level between 6.5 and 7.0. You can check with an extension agent to make sure about your particular grass, however, normally grasses will fall in this pH range. Add a sulfur-based fertilizer to increase the acidic level if the reading is above 7.0. If it is too acidic, or has a reading above 6.0, add lime.
Water your lawn more often during drought conditions if you are allowed. Most cities will put water restrictions in place during droughts, in which case you cannot do anything about the issue, but if you are allowed to water, add an extra day to your watering schedule.
Aerate compact soils. All soils compact over the years, and some soils such as clay are worse than others. Use and aerator on the lawn to loose the soil back up and mix in some air. If you plan to add aeration to your lawn maintenance, schedule it before fertilizing and overseeding.
Try not to kill needed micro organisms by limiting chemicals if possible. Use vinegar to spot treat for weeds, or corn gluten if you need to spread weed killer over a large area. The high acid content will kill them. It will take a year for the herbicides to dissipate. For highly compact areas, or areas where grass just isn't growing no matter what you do, till the soil and add some materials. Add mulch and other natural organic material to the soil to add nutrients and encourage micro organism growth, and sand to help with compaction. Leave clippings on the lawn to help with fertilization and help improve hard clay soil over time.