Comments on movement and breaking up your outline by sitting in front of grass/trees/bushes are spot on. Also, avoid skylining yourself. Any shine from your equipment should be masked. I think camo is good, mostly because it tones down shine and breaks up a solid surface. A face mask is excellent if you want to have game get close to you. The shine of a human face (or arse end)

is disturbing to game. As an aside, a couple of helicopter pilot buddies said that it was always easy to spot folks having a love session out in nature by spotting the shine of a guy's bare backside

. They would then pull in and hover for a laugh w/o too much concern about a low flying complaint being called in.

Ditto on spotting an upturned face from the air.
At close range you should avoid making eye contact with an animal who has not got your scent or is not otherwise alarmed. Your hands also stand out, even without movement, so gloves are a good thing. I've had many close encounters with game by watching the wind and paying attention to all of the above. Geese and old Tom Turkey are especially sensitive to movement and shine. When turkey hunting or muzzle-loading for deer I like to wrap the barrel of the gun in burlap to break the outline and avoid shine. I've also used a piece of pipe insulation with dull camo tape on it clipped around the barrel, so that your shotgun rib or sights can still be seen.
I remember an old army training film titled "Why things are seen". These primarily included shape, color, shine, silhouette, and movement.

Self-evident perhaps, but we always used to like to check our cam and concealment from the "enemy's" point of view.