With the proper alignment, you'll be able to run more easily and prevent injury. Here's a tip for naturally straightening up while running and avoiding poor posture: let your breath and gaze correct you.Photo by William Murphy.
The key to overall posture is how you hold your head, according to Runner's World. One way to make sure your neck and back are aligned is to look ahead (rather than down), scanning the horizon as you run.
You'll also want to "run tall", with shoulders low and loose. If you find yourself slouching or your shoulders high and tight, take a deep breath to find yourself naturally straighten.
The Perfect Form [Runner's World]