I position subs in a fairly complex way, but the method gives me the best (flattest) bass response, which is my goal. It depends on measuring equipment like REW + UMIK1 or the Dayton OmniMic2 system.
Think about the possible positions of the sub.
In each position, measure combined frequency response of the main speakers and sub, 15-300 Hz, while adjusting the sub's phase control (if it has one) or polarity switch for flattest response at that position. (The adjustments will be different in different positions.) Save a graph.
When you have done, some patterns will emerge. There is usually one very deep null in the audio range that can be filled with the sub in only one or a few positions. There you have it! Or if you are blessed with a room that does not generate a deep null, pick the spot that has the flattest response overall.
People have all kinds of theories about how to do this based on room dimensions, etc., without measuring the actual frequency response. The problem with such approaches is that wall materials and furnishings affect the results strongly. In addition, setting sub phase correctly has a large influence on overall flatness.