Start in Program Mode
I've already told you why you want to start in program mode. I once worked at Macy's assisting Stanley Ciconne, the fashion photographer. The second Stan saw the darkroom lights come on he'd yell, "Ken, do we have images?" He was wise enough to know that as long as there were images, we were closer to a finished ad than without them. As long as you start with your camera's mode dial set to P (and you've taken your lens cap off and turned on the camera), you will be able to get an image.
When to Switch to Manual Mode
Switch to manual (M) mode when you know you don't want the camera to reset the exposure because you just point the camera at something in the scene that just happens to be brighter or darker. If I've set my camera in P mode and I have a few seconds to check my exposures, I play back what I've shot most recently, pick the image in the series that I think has the best exposure, then press the Info button on the back of the camera to see what the actual exposure was. I can then switch to M mode, set the aperture and shutter at the successful settings, and then reset the dials to favor shutter speed (because I want to stop motion, create a blur, or make sure my old hands aren't shaking the telephoto lens) or aperture (because I intentionally want DOF to be either extended or narrowed). Once I've done that, all my exposures for that "scene" will remain consistent and I will get the results I expect.
There's another time you need to switch to manual mode: when you're shooting a panorama or want absolute control over shutter speed for bracketing a high dynamic range (HDR) exposure (see Chapter 11).
When to Use Aperture Priority
Use Aperture priority (A or Av) to make sure the aperture setting stays exactly where you set it, no matter what happens in front of the camera. You can also use this for bracketing in HDR mode, since only the shutter speed will change. Note, however, that there is a greater chance that something unexpected (such as someone popping into it wearing a white shirt or dress) could make the changes in shutter speed less predictable.
When to Use Shutter Priority
Use shutter priority when you know you want to do one of three things: