Think about it: when you're listening to music or movies most of the sound that reaches your ears doesn't directly come from the speakers. You're hearing a lot of sound reflecting off the floor, ceiling, walls, and other objects in the room. Speakers "play" the room. It's an analogous to way light illuminates a room, the source of the light may be the light fixture, but most of the light you see is reflecting off the surfaces of the room. With speakers we're trying to reduce reflections where we can to maximize the amount of direct sound we hear.
Stereo speakers should be placed with their tweeters at your ears' height when you're sitting down. The left and right speakers should be exactly the same distance from your prime seating position. Keep them 18 or more inches away from room corners or large pieces of furniture.
Placing stereo speakers up against or close to the wall behind them will "reinforce" their bass output, but some speakers produce boomy or overly thick bass when placed too close to the wall. Moving them further out into the room may help smooth their bass response. Experiment with speaker-to-wall distance while playing bass-heavy music; having a friend move the speakers while you're listening will speed the process. Beyond bass quantity, the overall sound quality will be affected by proximity to walls and large pieces of furniture. Try listening to well-recorded music with the speakers directly up against the wall, then one foot out from the wall, then two feet out from the wall. You may be surprised by how much the sound changes with each speaker move.
Satellite or bookshelf speakers can be placed on floor stands, mounted on wall brackets, or placed on furniture. If your room has a lot of uncovered windows (or mirrors) and hardwood or tiled floors, it probably sounds too reverberant. Large, sparsely furnished rooms (more than 300 square feet) with exposed floors and lots of glass are the toughest challenges, but adding thick rugs and heavy drapes may work wonders.
The optimum listening distance for iPod speakers varies from model to model, but most sound best when the listener is between three and five feet away from the speaker. Some manufacturers reveal the optimum distance in the speaker owner's manual. If not, experiment for yourself.
Home theater systems' front three--left, center, and right--speakers will also sound best with their tweeters elevated to the same height as a seated listener's ears. If possible, try to keep the center speaker near the same height as the left and right front speakers. Surround speakers should be positioned a little higher than the seated listeners' ears. For more info check out Dolby's Web site, which offers very specific placement recommendations for 2.1, 5.1, and 7.1 channel home theater systems.