One of the many great things about walking is that everyone knows how to do it. But if it's your main way to exercise, a few form tweaks will help you burn extra calories.
Stand up straight, as if a string is pulling your spine upward. Don't lean forward as if you're heading into a stiff breeze.
Look straight ahead, not down at your cool new shoes.
Relax your shoulders, bend your elbows at a 90-degree angle, and move them back and forth (not across your chest) as you walk. This helps propel you forward and gives your arms a light workout. Don't bring your hands up near your face or develop a punching motion; just drive your elbow back at waist height.
Keep hands relaxed, in a loose fist (don't clench or flap them around).
Your stride should feel right to you. It won't feel fluid if your steps are too short or too long. Aim for a smooth, rhythmic, natural pace.
Once you hit your natural stride, your foot will roll from heel to toe. Then think about pushing off with your toes -- it helps you walk faster and burn more calories.