It depends a lot on your speaker, and your room acoustics as mentioned above.
The sub is by far the hardest to place, as the accuracy, definition and level of the bass frequencies are most affected by the room. The 'right' place for the sub is literlally the position in which is sounds best in the main listening position.
The front speakers should generally be angled in toward the listening position, but to what degree will depend on the dispersion characteristics of the speakers.
Surrounds should be above head height (2-3ft), and there exact position will depend on their configuration - direct radiators, dipoles etc.
Good resources and guides can be found in a few places, but I would trust the experts at Dolby, or DTS (just google to find their websites) - together with any guidlines your speaker manufacturer gives you.