As spring arrives (and leaves, and teases, and retreats...) more and more people are getting running again. If you are just starting out, learn correct form first, if you are just starting back, take this opportunity of rebooting to check out your stride and evaluate your form, if you're a year-rounder, take a spring cleaning look at yourself and double check for loss of efficiency and/or possible flaws in mechanics that would make you more prone to injury.
Conduct a lengthy mid-flight checkup and see how you're doing.
Stand up, now really Stand up (tall): pretend there's a string coming out of the top of your head, reach up, grab it and pull up, lifting all of your body Tall and straight.
RELAX: relax everything, loose fingers, hands, wrists, elbows, shoulders, and neck
Check your leg movements: March in place - your feet land directly underneath your center of gravity (under your hips), you are Lifting your legs off the ground (with your hamstrings, quads, and butt), Not pushing yourself off the ground (with your calves)
Bend your knees. Now bend them even more, and a little more, and keep them bent. Knees should stay bent the entire time, even (especially) when you land. Your knee should Never lock out - you have God given leaf springs, use them
Land on the ball of your foot and immediately roll flat (think about landing perfectly flat)
Bend slightly from the Ankles (Never bend at the waist)
When you get tired, the first thing most people do is start to lean forward and their shoulders come forward and in, constricting your lungs and further exacerbating your breathing. Stay tall, head up, shoulders back, chest high
**Remember the principles: straight body posture, everything (toes, knees, hips, chest, shoulders, arms, and head) pointing and moving in the direction you want to be going, loose, loose, loose, lift your foot off the ground using your big muscles (the "Pull" in Pose) instead of springing into your next stride with just the use of your calves, land lightly on the ball of your foot and roll flat, breathe In the mouth and out the mouth, smile ;)