Now is the time to precondition your lawn so it can stand up to summer heat. Fertilizers create lush growth susceptible to the heat. And herbicides can also stress grass. Gradually raise the cutting height on your mower by 25 to 50 percent as temperatures increase.
Longer grass encourages deeper roots, and it shades the soil better.
Also be sure to mow as often as needed so you never remove more than a third of the grass blade at a time. Water early in the morning. Do it thoroughly enough so water soaks deeply into the soil - light, frequent sprinklings cause shallow roots unable to provide needed moisture. Apply 1 in. of water per week.