The links posted by Mozilla are very useful and very scientific. Great tips for everyone. They greatly stress the relationship between the loudspeaker and listening position.
For the people who do not have the benefit of dedicated listening rooms, and just plain living rooms, hope this helps.
The distance or proximity of the speakers to the walls affect the amount of bass. The closer they are, the louder the bass. SOME speaker designs like nearfield bookshelf monitors were designed to placed near the rear walls.They depend on the backwave to extend their responnse.
Most loudspeakers get better soundstage depth when they are placed a little farther out into the room (of course not in the middle of your listening room). Sometimes you wish you can follow George Cardas' suggestion, or Audio Psychic's room mapping technique unfortunately, many living rooms and bedrooms don't accomodate speakers far out into the room. If you're forced to place it close to the wall, you can use absorbent panels or bass traps, to cut down on the boominess.
Your tweeter's distance from the floor matters a lot, so vertical placement is also a consideration. Since tweeters are like pistols, and woofers are like shotguns, putting them on-axis to your ears will produce the best response. Sterephile recommends "32" to 40" from the floor should coincide with typical listening heights". So you can either invest in a pair of speaker stands or a gas-lift chair to get the best response from your speakers at the right height.
Prior to the days of commercially available cheap laser pointers, I taped small Maglites to aim my carefully positioned speakers to put them in the right direction (Primitive!). To adjust toe-in, I use in-pahse/out-of-phase pink noise tracks from either Ultradisc Alan Parsons & Stephen Court Sound Check, Sheffield MyDisc, IASCA Competition CD, XLO Test and Burn-In CD, or even the Video Essentials DVD. I find it easier to use Pink Noise instead of music for toe-in adjustments. I start with out-of-phase signals to adjust speaker distance, and in-phase for center focus.To audit my settings, I reverse the tests . It takes me around 2-5 minutes to get it right depending on the room. Once I do, I check it this time with in-phase/out-of-phase voice and mono music.
Non-rectangular rooms are a bit tricky to fix. Some have chambered walls like bay windows, some even have half a back-wall, some setups I handled restricted the speakers to a small corner of the living room. With a great deal of patience and adjustments to the furniture, you're bound to get it right.