Posture is key to having a successful run and preventing injury.  Our goal is to reduce the effects of gravity and we do so by having our:

Whole Body:  Relaxed & Balanced
Head:  Above Shoulders & Lifted
Shoulders:  Back & Loose
Chest:  Forward
Arms:  Close to the Body; Relaxed
Hands:  Loose (you should be able to hold a pototoe chip between your thumb and forefinger without breaking it)
Abs:  Engaged
Hips:  Forward (like sitting); In Line with Shoulders
Knees:  Low Kick Back (no dramatic spring)
Legs:  Avoid Over-Stride
Feet:  Light & Low (stong kick off/avoid bouncing)