Fast Shutter Speed. When a subject is moving fast, you need a fast shutter speed to freeze the action. The faster the subject is moving, the faster the required shutter speed. I photographed the bald eagles using a shutter speed of 1?2000 sec.
Accurate Focus. Right-on focus is necessary to get the sharpest possible shot. When a subject is moving, set your camera to AI Servo focus (Canon) or Continuous Focus (Nikon). These settings (AI Servo, in this case) allow your camera to track a moving subject right up until the moment of exposure. How cool is that! When a subject isn't moving, set your camera to one shot.
Depth Of Field. Pictures with shallow depth of field can lack apparent sharpness. In this photograph, the inflatable boat in the foreground, the boat in the middle of the photograph and the background are all in focus, which was my goal for the photograph. To achieve that goal, I set my 70-200mm lens to 85mm and shot at ƒ/5.6.
ISO. At high ISO settings, you'll get more noise in an image than you'll get at low ISO settings (although with some of the newer digital SLRs you can shoot at ISO well over 1000 and still get very clean images). Noise can make an image look soft, even if it's sharp. My advice is always to shoot at the lowest possible ISO setting. That may be quite high if you're handholding a shot in very low light. Still, go for the lowest setting. I took this handheld shot at sunset with the ISO set at 500.
Lens Hood. When direct light hits the front element of your lens or filter, you get lens flare. At its worst, lens flare shows up as a bright spot in your picture. To a lesser degree, lens flare can make a picture look soft. Always use a lens hood, or "flag" your lens with a hat or hand, to keep direct light off the front of your lens.