Where to Put Your Television
Ideally, you'll want to find a spot that's not directly across from windows and doors, so as to reduce the amount of glare you'll have on your screen. While anti-glare technologies are getting better, any viewing experience is going to be tainted by direct sunlight streaming across your screen. Also, allow room to arrange seating. If possible, you'll want to arrange a 30-degree viewing distance from your TV, or around 1.9 times the screen width.
Where to Place Your Mount
For the most comfortable viewing, you'll want your television to hang about 40 - 45 inches from the floor, or right at eye-level when seated. While ideal, this arrangement isn't always practical in wall-mounting. However, as long as the bottom of the television is no higher than eye level when seated, and the top of the television is no higher than eye level when standing, you should be within a good range for comfortable TV viewing. You'll also want to consider the position you'll be in while viewing. If you're laying in bed watching television, your viewing angle will be different than if you're sitting in a chair. Keep in mind that you will want to be at about a 30-35 degree angle, and your mount should work perfectly.
Some Things to Remember
Always make sure the mount is perfectly level, as a botched job is hard to fix later and can damage both the mount and, ultimately, the TV.
Always follow the mounting directions exactly, and make sure you double check your measurements before getting started.
Check for any electronic cable or wiring before starting by purchasing an inexpensive electricity cable locator at a hardware store.
If installing into drywall, make sure to screw the mount into a stud, to maximize stability.
Find someone to help you with the installation process. Your TV will have a much better picture when it hasn't first been dropped on the floor.
If you keep these things in mind, you're sure to have a perfectly mounted television in no time!