I leave early and take it slow. I'll get there when I get there.

My water rule is 1L every 5 miles in the heat. I'm not shy about going into coffee bars, buying something cheap, and asking them to fill up my water bottles. I'll detour to go past libraries, with their nice cold water fountains. My second water bottle of the day always gets a Nunn or similar in it.

+1 on wearing cotton. Wicking fabrics make me feel hotter.

If I know I'm going more than 10 or 15 miles the next day, I avoid alcohol the night before. And if I'm not feeling well for any reason, I take the bus or ask for a ride.