The most important thing to remember about running in hot weather is not to run more than you are used to running or to put it differently, do not over do it. It you are running four or five days a week all through the spring than when it warms up you will have gradually acclimated yourself to the hot weather. If not you should take it easy on hot days and listen to your body and constantly monitor how you are feeling.
The next thing to consider is hydration. You can actually loose several pounds in water weight when running in summer. Water is the best and at room temperature, when it will enter your system fastest. Any non-alcoholic, non-Caffeinated drink is next best but will take much longer to enter your system. Avoid alcoholic or caffeinated drinks since they are diuretics, which cause you to loose water. Depending on the individual the effects of caffeine can remain for up 12 hours.
You can also choose your time of day. Early morning is coolest and late evening is next although on some hot summer days it never cools off in the evening. Much depends on what your are accustomed to

How To Know When To Quit.
If you stop sweating, have a rapid pulse, hallucinations, confusion or disorientation then it is time to sit down and rest. If you are out on your own don’t be afraid of walking home, there is nothing wrong with that. The hot weather can affect everyone. Just gradually accustom yourself to it. If you see someone running in front of you who is weaving around and running erratically please stop and see if they need help. They may not even be aware that they in trouble. If they know where they are and do not want help then you can finish your race knowing you did the right thing.