Put the sub where you normally sit (yes, on the couch), and play music with different kinds of bass. Crawl around on the ground, around the walls in the room. Listen for the best - as in loudest, smoothest, not "boomiest" - bass. That is where your sub will work best.
Every room is different, so this is the best way to find the best spot for the sub.
That being said, a good starting point is a front corner, or up to 2-3 feet out of the corner, along the front or side wall. And make sure your seating location isn't against the back wall - horrible for bass (will be very boomy and bloated).
Remember that you will have to re-calibrate the sub's level if you move its location.