Always go to the store armed with measurements of your space at home -- if it doesn't fit when you get back, you'll want to smash things cry.

The no-brainer option is to use a built-in stand on a table or entertainment center, especially if you go with one of the heavier sets like a big DLP. [Kurtis: I might be inclined to use a wall mounting system for an LCD or Plasma, but you'd have to spend big bucks to get something substantial enough to hold, and articulate, a ~70 lb. DLP.]

If you're going for a wall mount, know what the viewing space will look like before choosing a mount. Some offer tilt / swivel in order to better fit a space, but you may not need these extras. My personal experience was with a fixed flat mount and it worked flawlessly once I was finished (following a few hours of blood, sweat and obscenities).

One important thing to keep in mind is that the TV should be placed at a comfortable viewing distance and angle. There are a few big standards (THX, Society of Motion Pictures and Television Engineers) that give very specific recommendations, but you don't have to stick to them explicitly.

A good guideline is to look for a TV with a screen width (not the advertised diagonal size) that is between 1/2 and 1/5 of the distance between the seats in your viewing area and the TV's location. It's also a good idea to keep the TV around eye-level in your normal viewing position. While the TV might look cool over the fireplace, you'll probably find that it's a literal pain in the neck.