It is generally believed that the bass you hear below approximately 80 Hz is non-directional. This means that you can point the loudspeaker in any direction and the sound will still reach the listener's ears. Since much of the bass that subwoofers produce is below that frequency, you can place the subwoofer almost anywhere in the room. This is the opposite of full-range speakers, which have just a few placement options in a room that allow them to sound good, since they must be positioned for the best combination of imaging and tonal balance. Putting a subwoofer in a corner of the room may cause the sub to sound louder. If your subwoofer is a ported design, keep it at least twice the diameter of the port exit (probably 6-12 inches) away from the nearest wall, so that air flowing out of the port is not obstructed. If the bass seems too "boomy", you can fine-tune the sub by moving it farther from the wall until it sounds smooth to your ears.
The best way to find the ideal place for your subwoofer is to hook it up and put it right where you will be sitting in the room. Set the subwoofer to level, low pass and phase. Play something with consistent, deep bass and move around the room on your knees; this way your head is about where it would be when you are seated. The spot where the bass sounds best is a spot where you might put your subwoofer. You may find more than one location.
Corner placement of the subwoofer typically yields the loudest output (highest efficiency). This does not mean that it will always sound best in this position; experimenting with placement is always suggested. It's also important to have the sound from the sub reach the listener in sync with the sound from the main speakers; otherwise the sound may not blend properly. You should not be able to hear your subwoofer as a separate entity—it should seem that your main speakers go deeper with greater impact and authority.