Coaches have multiple goals when it comes to training. They need to improve the fitness of their players and their skills. Fitness comes in many forms, there is speed, endurance, power etc. Which of these are developed depend on what the player needs and the requirements of the position. Here are a few ways to improve speed:
One way of improving speed when running is to make sure you are running properly. There are guides that can be found online that will help check form when running. Incorrect form and technique are very important. Getting the mechanics right can remove seconds from your 100m time.
Once you work out any issues with your mechanics, then you can look other ways to improve speed. The next thing to try is mobility work. Mobility work is usually not needed for younger players but older players who spend most of the day seated can have their flexibility and mobility compromised. Weekly mobility work will go a long way to restoring that movement and allowing the player to make the most of his mechanics.
Strength training is another must for improving speed. Younger players can start with bodyweight squats as they might be too young to lift weights. Older players on the other hand can utilize the gym or weight room to build strength and muscle. This in turn will give them a power advantage and speed. Some examples would be Squats, Deadlifts and Kettlebell swings.