Turn the amplifier on and send the audio signal to the subwoofer while it sits in the middle of the room.
Crawl around the room and listen for where the subwoofer sounds the best. Make sure to follow the perimeter of the room on all sides. Listen for clarity and strength, taking note as you continue to crawl throughout the room.
Place the subwoofer in the location where the sound was the clearest. This area is going to give the best bass response and maximize the sub's power and auditory potential.
If you are using the subwoofer for home theater applications, try to place the speaker in a corner of the room. The subwoofer can use the surrounding walls to focus the frequencies directly to your listening area.
If you are using the subwoofer for studio applications, avoid placing the speaker in corners of the room. Over-exaggerating the bass signal can give a false representation of your mixes and should be avoided.