This is an excerpt from Coaching Cross Country Successfully by Pat Tyson & Doug Binder.
You want to identify several key elements of effective running form right away. You want your athletes to run tall, with a long, upright posture that maximizes the length of their levers and opens up the lungs (see figure 7.1). That said, you don't want them to look unnaturally rigid or tight, either. You want them tall but also relaxed.
I tell kids to carry a feather between their index finger and thumb. This promotes the idea of running light and loose. You don't want tension in their arms and hands. But you also don't want hands and wrists that are floppy (see figure 7.2).
Arms should be held no higher than the shoulders and no lower than the hips (see figure 7.3). Most kids have a natural arm motion that is comfortable, and I tended not to correct it unless it became a problem. The thing to watch for is tightness in the arm carriage. It's a sign that the athlete is carrying tension and needs to loosen up.
Good arm motion.
Proper running form, arm carriage, and upright posture are easy to maintain at a jogging pace. It's much more difficult at the end of the 5,000 meters when you are trying to lift and sprint as hard as possible.
As a coach, you want kids to be strong enough to run fast even when they are tired. On top of that, you want them to run fast and remain relaxed. Focusing on arms to drive the legs with a little more gusto can be a big help at the end of a race. Powerful arm drive coming down the homestretch and relaxation in the face are sure signs of closing strong.
The best way to run hills is to slightly lean into them, pushing the center of gravity out front. My rule of thumb is "nose ahead of toes." Also, runners should rise onto their toes, which tends to happen naturally if they are leaning into the slope. Occasionally, you might want your athletes to attack hills in order to gain ground on the opposition or pull away. But in general, my advice is "pace it, don't race it." Otherwise, there's no energy left at the top of the hill. Strong, active arms can be helpful going up.
Coming down the hill, the foot strike naturally transitions from the toes to the heels. It's the first place the ground makes contact with the foot. Reaching out with the feet and lengthening the stride will help to gain more ground and prevent any tendency to lean back into the hill. Leaning back will apply a braking force to the running stride, which will slow the runner down. Runners should use their arms in a normal fashion but carry them a little lower. Runners should let things go a little bit on the downhills as long as they can remain in control. This is a place on the course where they don't want to hold back unless there are hazards or concerns about falling. Gravity is their friend.
Arm Swing and Racing Stride
At race pace, a runner should strive for efficient short strides that are dictated by arm movement. Generally, the hands move down to the hip and back up near shoulder height. The arm swing is smooth and concise. Watch a video of Matthew Centrowitz, Galen Rupp, or Shalane Flanagan, three current athletes who have efficient strides and arm motion.
Running hills and stairs can help make a stride more efficient by training the muscles to generate more power in each stride. Generally, the legs will follow the arms. Short efficient arms will create an efficient stride and prevent over striding. A longer, more powerful arm swing will open up a stride, helping to create a longer sprinting stride useful at the end of a race.
Aim for midfoot or ball-of-the-foot contact with the ground. An athlete who runs on the front half of the foot goes faster. Landing on the heels and rolling from back to front is a slower process and less efficient. Instead, runners want to feel a little bouncy and frisky. Again, hill running is a great way to promote running on the toes.
Athletes who run on their toes are pushers, lightly pushing themselves away and forward from the surface. Runners who are less bouncy and make contact farther back are pullers. Some excellent runners are shufflers who fit into this category, and sometimes biomechanics dictate one style over the other. But in general, the pusher is faster than the puller.