One of the biggest mistakes beginners make is to employ poor posture when they run. One common mistake is to lean excessively. Unless you're racing the 200 meters (or shorter), don't lean too far forward. Doing so throws off your balance and leads to overworking your back and thigh muscles.
Proper posture begins with the correct body angle. To get a sense of this body angle, stand up straight against a wall. Your core should be engaged, your chest should be up but not out, your shoulders relaxed, and your buttocks pushed firmly back. Now lean slightly forward from your ankles so that your shoulder blades lift away from the wall. This is the posture with which you should run.
It allows for proper breathing, prevents you from overleaning, helps you to lengthen your stride, and makes knee lift easier. Good posture ties your whole body together. It even helps you to have a more efficient footstrike.
An Austrian actor named F. M. Alexander, who lost his voice because he was clenching his throat muscles, developed a posture-enhancing technique known today the world over as the Alexander Technique. Beneficial for anyone from daylong computer users to professional athletes, the Alexander Technique, by helping you become more aware of your posture, takes stress off your body and enables you to move more freely.
In thinking about your posture as you run, you should consider where your hips are when your foot hits the ground. The optimal foot placement occurs when your foot strikes the ground under the center of gravity of your body. A line from your ear through your shoulder and hip should end up at your ankle. You should run with a straight posture and a slight forward lean. Look ahead to where you are going, or look at the ground 10–20 feet in front of you.
As you are running also be careful not to stick out your chest, since doing so increases tension in the muscles in your upper back and neck. Your shoulders should be relaxed and your core engaged. Even as you focus on good posture, remember to relax. You want to run standing up straight, but comfortably so.