Entire books have been written about this topic, so the following advice should just be considered as a starting point; exact positions will vary depending on your room acoustics and choice of loudspeakers. The following advice is suitable for either stereo speakers (ie. 2 channel) or the two main front speakers in a 5.1 or 7.1 channel surround sound system. Good speaker positioning improves the sound quality and stereo image, so that instruments sound more lifelike and appear to come from different parts of the stage (as they would sound at a live concert).
As a starting point, locate the "tweeter" or small high-pitched driver in the speakers (Just play some music with plenty of rythym and listen for where the cymbals appear to come from!) These drivers should normally be positioned at about ear level when you are sitting in your usual seat (or standing up dancing!)
Measure from your usual listening position to the front wall of the room where the speakers are placed (ensure that they are in front of you, not to the side or behind you) This distance should be roughly the same as the distance between the speakers.
Now look at the back panel of the speakers. If there is a hole in the back (known as a bass port) or a grille cloth on the back (covering a bass driver), then the speakers should normally be placed away from the front wall of the room. If neither holes nor grilles are present, then the distance from the front wall is less important. The distance the speakers should be placed from the wall for best effect will vary, but 50cm (0.5m) is usually a good starting point for most speakers. Listen to some music you know well with a clear bassline and try various positions to see which gives clear, realistic bass notes that fit in well with the rest of the music.
Finally, ensure that the speakers are solidly mounted on suitable brackets or stands (for smaller speakers) or securely placed on the floor (for taller speakers). Try pointing the speakers directly at your listening position, straight at the back wall and then try them angled inwards to point at a position about 50cm in front of your listening position. This will alter the positioning of instruments "on stage" and you should be looking to hear the lead singer clearly in the centre, with backing vocals and instruments spread evenly between the speakers.
Above all, there is no "right" or "wrong" way to position your speakers, so feel free to experiment and find out what works best for you.
Steve at Sounds Heavenly