Teddy is my exuberant cocker spaniel puppy who is full of energy, and yesterday I wanted to get some shots of him running through the tall grass in my backyard that didn't get cut while I was traveling for a month. He is so short that he has to jump constantly just to see ahead of him, and I had my wife throw a tennis ball such that he'd run toward the camera. I used AI servo to track his motion, and the 300mm f/2.8 telephoto allowed me to capture a fairly tight composition. I had to take a lot of shots on motor drive to get a few good ones, but that's par for the course when it comes to shooting action.
My settings were: 1/1000th of a second (specifically to freeze Teddy's flapping ears), f/5.6, 640 ISO.