Movie soundtracks are made for rectangular spaces, and a 10-to-7 side-to-front ratio is ideal. The perfect room would also have a ceiling that's higher than its width but shorter than its length. At the very least, avoid a perfect cube (sound waves might cancel each other).
For a conventional TV, set up your seating area at a distance of 2-1/2 times screen size. For an HDTV, it's twice screen size.
Surround sound speakers should all be the same size and placed at ear level. Left and right speakers should flank your screen roughly 8 feet apart. Put your center speaker directly above or below the TV. Left and right surrounds should go behind the seating area but oriented toward it.
Two small subwoofers are better than one large one. If you have just one, place it near your screen. If you have two, put one near the screen and the other behind you.
The room should be carpeted, so sound isn't deflected around the space. Drapery or shades also suppress deflection.