Tonality is important even if you're shooting in color; Adams's Zone System helps ensure correct and appealing tones. The zone system is super-simple to use with digital SLRs.

When you look at your photos on your LCD after taking them, you can view a histogram of the tonal values -- a graph showing the number of pixels in each tone, from black to white. Conveniently enough, your camera will cause the true blacks and white to blink as an alert to you -- but true blacks and whites are your friends. Use them. When you compose your shot, look for elements you'd like to be entirely white or entirely black. Place those elements, in your mind, in Zone 0 (black) or 9 (white). Shoot your image as you think best, then look at the histogram. Is the white you want white as white as you want? Increase exposure 1/3 of a stop for each zone your white is away from 9, or decrease by 1/3 for each zone your black is away from 0. Shoot again, and you'll get the image that has the tones you want.

You can use the histogram for even more than that -- look at your scene and try to visualize what the distribution of tones should look like. Are there more darks? Then you'd probably find yourself with more tones in Zone 3. Pretty bright? A lot probably in Zone 7. Your histogram should reflect this, with a "hump" around that zone. Does the distribution look right? If not, overexposing will shift the distribution to the right (more tones in higher zones), and underexposing will shift the distribution to the left. You can't change the shape of the distribution, but moving it is trivial -- and, because you can see the histogram right after you take your first photo, you can make the adjustment and get your tones right, right on the spot.