Although running style tends to be roughly preordained by your innate and individual biomechanics and shouldn't be tampered with drastically, minor corrections can make a big difference in performance and injury prevention.
Give yourself a casual test or have a buddy give you the once-over while running. Health clubs often have treadmills positioned in front of mirrors, which can be an excellent way to check yourself out.
If you think your own running form could use some attention, find a trainer or strength and conditioning coach to help. If you've been running injury-free for years, it probably doesn't make sense to drastically change your gait.
However, most of these tips can help you to run with more ease and less tension, saving your energy for speed and endurance.
Head Position:
Look straight ahead, neither up nor down. Head should be relaxed, eyes on the course about 10 feet ahead. Fatigue can cause shoulders to raise up and the head to tilt back. Be conscious of your position.
Body Angle:
Run tall. Imagine a plumb line dropped from the top of the head to the arch of your feet. Plumb lines follow the pull of gravity perpendicular to the ground, and so should your body.
Arm Action:
Never carry the arms high on the chest. Arms should be relaxed at about a 90-degree angle. Swing should be like a pendulum from the shoulders, elbows down, the arch extending from chest high to the seam of your shorts. The upper body should not twist from side to side.
Hands:
Keep your hands relaxed and your wrists loose. Cup your hands and touch the thumb to the forefinger.
Knee Action:
Let your leg come forward naturally swinging with the weight of the leg. Knees shouldn't be lifted beyond their natural swing.
Footplant:
Here lies the greatest controversy in running biomechanics. Although some will argue for a heel-first landing, especially for older or less accomplished runners, many authorities call for you to land on the lower part of the ball of the foot, drop the heel, and push off the ball of the foot. The faster you run, the more tendency there is to land on the lower ball of the foot. Do not turn your feet out.
Stride Length:
Don't overstride, this is the cardinal sin of running. Avoid reaching for the next stride. Run tall with a low forward knee lift.
Relaxation:
Avoid unnecessary use of energy through tension and stress originating in the face, hands, or arms. Save your energy for running with a relaxed, efficient form. It pays off.