Our muscles and tendons are elastic in nature. Some people seem to naturally have a little more of this elasticity than others. Increasing this elasticity will benefit running performance. When the foot strikes the ground (if we're landing on the forefoot like we should an not on the heel) the tendons and muscles stretch. And like someone snapping an elastic, the muscles and tendons also snap back into place giving us free energy that the body doesn't have to produce. The longer the foot is on the ground, the more this energy dissipates and the more energy we have to produce. This is why we want the foot's contact time with ground to be a quick and short as possible.
In order to increase the elasticity in our legs we do plyometric (jumping) exercises which quickly load and unload the muscles and tendons. My goal here is not to go into all the different plyometric exercises we do, but rather to discuss their appropriateness for youth athletes. Many believe that because of the intense stress that some of these exercises put on the musculoskeletal system than they should be avoided until the athlete is physically mature. Other say that because musculoskeletal system is more "plastic" in nature when we're young, that youth is a good time to begin such exercises. With this in mind, we do perform these kind of exercises, but we do them cautiously and increase the load very slowly over time. Always be sure to warm-up well and ease into plyos. I recently heard of a middle school x-country team begging their warm-up plyos. This would be very risky and would not produce the full benefit as our muscles are more powerful and flexible their temperature is raised from a good warm-up first. (In 2014 I'm starting to see sources suggesting that you can be a little more agressive with plyos at a younger age, and that they be even be beneficial in injury prevention.)
I'm continually surprised when I see coaches that are still stuck in the stone ages when it comes to stretching. The current trends/info of static vs. dynamic stretching have been out for some time. I learned about this years ago and felt dumb that it had taken me so long to catch on. The idea is that if we're trying to improve the elasticity of the muscles and tendons, we don't want to over stretch them with static stretches (the traditional stretch and hold for 10+ seconds) pre-workout. We use to think that this would help prevent injuries, but recent studies suggest that static stretching also over stretches the supportive tissues around the joint making them more prone to injury. All that is really necessary (and recommended) is a few dynamic stretches (moving stretches such as leg swings) that take the athlete through the range of motions applicable to the sport. And be sure to be well warmed-up before performing any stretches. When the temperature of the muscles is raised, they become more flexible and elastic in nature. This takes more than the token "warm-up lap" you see many youth programs doing. I think it takes at least 10 minutes of easy exercise to actually get the desired raise in temperature in the muscles. I also still believe in wide range of static stretches post workout for reasons of maintaining flexibility.