Keep at Eye Level
According to Crutchfield Electronics, the ideal flat-screen plasma or LCD TV mounting location is directly at eye level. This is approximately two to three times the screen size from your primary seating position. This will be about 3.5 feet off the ground with most couches or chairs.
Angle the TV Mountings
If you must mount your flat-screen TV slightly above or below eye level, consider finding TV mounting brackets that allow you to tilt the screen down slightly. Mounting directly at level with your eyes creates the clearest picture free from a washed-out look. Mounting the TV a foot above eye level angled downward will give you a clear picture while allowing you to lean back in your couch or chair.
Mounting at eye level two to three times screen distance from you is ideal but will possibly require you to reevaluate your TV mounting location or rearrange your room. Avoid direct light sources pointed at the screen and make sure to arrange your seating such that no one is too far to the side of the screen.