Drink plenty of water during the day. Don't worry about frequent washroom trips - you'll sweat most of it out. Note that water is preferred over carbonated beverages. Since you'll be drinking a lot, pop will cause you to feel bloated. As well, some pop contains caffeine which dehydrates and leads to hyperactivity - just what you want to avoid on a hot day.
Alcohol is to be avoided too.
It does not quench thirst, tends to increase the sensation of body heat, and also causes dehydration. Wear a hat. Heat exhaustion is a possibility, so a light- coloured hat is recommended to shade the head. Rest from walking often. Seek shade or an air-conditioned theater when overheated.