Listen to the sound your feet make.
If you hear a squeak, shuffle, skid or other such sound, stop what you're doing.
A distance runner should hear only the sound of her heel strike. From the heel, her footstep should unfurl as she rolls on the side or bottom of her foot, finally pushing off from the ball of her foot.
Listen the next time you're on the road as you pass other runners (or as they pass you, as the case may be). You will hear the difference as people shuffle or skid past as they run on their toes or the balls of their feet — or whatever unnatural, ungainly stride they use.
If you run in sand, snow, dirt, dust or gravel — something that allows for a clear footprint — look at your footprint. Your footprint should be clear and perfect (material permitting, of course — sloshy mud won't give you a clear print no matter how good your step).
If the footprint is smeared in any way, or if it is otherwise unclear, watch the way you place your feet when you run.
Now, while we're at it, try leaving off the earbuds when you run for a while. Without music or voices blaring in your ears, you can better hear vehicle and foot traffic around you. You will be able to hear when that tractor-trailer is not stopping in time to let you pass safely in front of it. You will be able to hear someone running behind you. You will be much more aware of your surroundings, always a plus for safety.
If you aren't attuned to your tunes, you will see things around you: the Cooper Mini that wasn't going to stop, the woman with the books who wouldn't have seen you, the geese feeding with their goslings at the pond, a Springsteen poster (not that all of us got tickets, *sigh*), the new shop on Main Street — the list goes on and on.
Rumor also has it that time will pass more quickly if you're not marking it, and a run will be easier if you're not constantly reminded about time (and the number of songs you hear while running marks time very efficiently).
Be aware of yourself and your step. Listen and look. It will make you a better runner — not to mention a more aware and interested (and interesting) individual.