Run at the coolest time of day. The National Weather Service advises moving strenuous activity to the coolest part of the day. In a Texas summer that usually means an early-morning start. Watch the heat index, not just the temperature, because humidity makes it harder for sweat to cool you.
Drink before you're thirsty. Drink plenty of water even if you don't feel thirsty. On longer runs, carry fluids or plan a route past water fountains, and check the trail map before you go, since water on the trail isn't guaranteed.
Slow down. Heat and humidity make every pace harder. On hot days, run by effort instead of by the watch, shorten the run or split it up, and build back up gradually when summer arrives.
Dress for sun. Wear lightweight, loose, light-colored clothing, plus a hat or visor and sunscreen. A local running store can help with lightweight apparel and hydration gear.
Know the warning signs. Heavy sweating, weakness, cool and clammy skin and a fast, weak pulse point to heat exhaustion: stop, get somewhere cool and sip water, and get medical help if you vomit or symptoms last more than an hour. Confusion or slurred speech can mean heat stroke, which is life-threatening: call 911.