Put that puppy on the floor, and as far away from where you sit as it can possible go.
Sub frequencies (under 100hz) are very long waveforms. For sound to propagate fully a waveform must complete a full cycle for you to hear it properly. At 60hz for example, the wavelength is about 18 feet long. Since that's around where the sub of most house/dubstep kicks lie, if you're sitting within 18 feet of the subwoofer you won't hear the bass properly (or at all).
A neat trick however is to use a large surface, like your walls, as a giant drum. If the subwoofer is against the wall (or close to it, you want that port, the hole in the sub, facing the wall) you can effectively halve the distance required to complete a full waveform (9 feet for 60hz) as the waveform will bounce off the opposite wall and complete a cycle in half the distance (or so you perceive)