You'll lose less cool air to the outdoors. When inside, close doors to rooms you don't use. Wear the right clothes. Loose and lightweight clothes can make a big difference in your comfort level. When air can circulate across you skin, it evaporates moisture and cools you naturally.
Insulation is just as important.
In winter, insulation helps keep warm air inside. In the summer, it helps keep warm air outside. Make sure your home has the appropriate amount of insulation in walls, attics and crawl spaces. Open windows at night. To take advantage of naturally cool night air, leave your windows open where it is safe to do so.