Back-button Autofocus
If you’re shooting with a camera that supports it, look into using back-button autofocus, which separates the autofocus system from the shutter button. Then, the shutter button starts and locks the light meter, and the AF button, pressed with your thumb, starts the autofocus magic.
Mid-range and pro Canon DSLRs will often have a dedicated button for this (“AF-ON”); the Rebels can use the AE-Lock (“*”) button for it. They all have a custom function that can let you select, basically, which of all these buttons do what — there are actually several ways to map different functions to these buttons besides the one I’m talking about here, one of which might be even more perfect for your work style. I can’t speak to Nikons from personal experience but I know they have this ability, and I assume other brands do as well — my EOS A2 can do this, so it’s not a new idea.
It takes a little bit of effort to retrain your thumb and forefinger, but not much. And for a lot of photographers, it’s worth it. If you think about it, having one button control the focus, the light meter, and the shutter all at the same time doesn’t make a lot of sense to begin with. A lot of times, the subject of your image is going to need be in focus and exposed properly, sure — but that’s no harder with back-button AF, whereas photos where that’s not the case are going to be much harder to manage without it.
Imagine trying to shoot a person against a bright, colorful sunset with fluffy clouds, and mackerel sky. You’re visualizing a mid-distance portrait with her silhouetted against the background, so you switch over to partial metering, because you don’t want the light meter to find some ugly middle ground by looking at everything: this scene is about contrast and color. So in aperture-priority mode* for shallow DoF, you meter off of the sky to bring her exposure down by two stops, full black, and AE-Lock it. Then you autofocus on the person, but the light meter wakes up again: your shutter speed drops to bring her exposure up higher than you want, the sky gets overexposed, and everything is either noisy or blown out. With back-button AF focus, you could have held the exposure with the shutter button, then focused with your thumb, without the light meter knowing or caring.
*Shooting in manual mode would also avoid this sort of problem, except of course that the light is fading fast in this hypothetical situation, and somebody’s got to keep an eye on the light meter, whether it’s you or the camera.
Another situation where this comes in useful is when you’re shooting in low but shifting light, where the AF drive has to work hard to find its subject in the first place. Imagine shooting a street corner on a cloudy day, with the street signs in focus and readable, and moving cars and pedestrians around it. You get the street sign in focus easily enough, but clicking the shutter button will send the AF hunting back and forth for whatever it thinks you want.
Sure, you can switch the lens AF off, but that’s is a workaround, and it’s easy enough to miss a shot because you forgot to switch it back on again. Back-button AF means you don’t have to think about working around slightly complicated, everyday focus/metering issues like this because they just don’t come up. It’s no harder than having the shutter button do all the work, and in many cases it’s just a lot easier.