Overspeed

Find a steep decline(better grass than a harder surface for safety). Start at the top of the hill and start sprinting as fast as you can toward the bottom. While this may seem easy because of the decline, it challenges your body from rolling over and falling. Going this quickly downhill will get your legs familiar with going faster than normal without losing control and balance. Practice this a few times a week and once you hit a flat surface, your speed will pick up.