This was nearly the last shot fired in an AI Servo burst sequence - before the puppy attacked me (playing of course). Photographing dogs running is very challenging. First, get down to their level (I was flat in the grass) use AI Servo AF mode to track the fast-approaching dog. A fast shutter speed is required to stop the action which means good light (this shot was taken in late afternoon on a sunny day) or a high ISO setting. Shooting with a narrow aperture will give you a higher focus-hit-rate as depth of field will be deeper. Shoot lots of shots and only keep the best.