Too many hunters are looking for an animal much bigger than a deer. We call this the "horse with antlers" mentality. Magazine photos and videos make whitetails seem much larger than they are. A mature buck stands about 3 feet to his back. The top of his rack is no higher than 6 feet off the ground. Everybody seems to think deer are going to be standing in the wide open, turned broadside with a bullseye painted on them.
That just won't happen. The trick is being able to pick a deer out in the thickest cover.
When you're looking for a deer, start low on the forest floor and look no higher than 3 feet. Particularly when still-hunting, hunters move through the woods with their eyes straight ahead, looking too high to see most deer. If a deer is bedded, its back is only a foot off the ground!
Look for horizontal shapes and movement, and also for contrasting colors -- the white around a deer's eyes and nose, the white throat patch, the white in the ears, the white inside the legs, the white on the tail, the black nose, the dark eyes, the black or dark hair on the end of the tail surrounded by white -- these contrasts are easier to notice when you train yourself to look for them.