Placing your subwoofer in a corner causes the bass waves to bounce off the wall in different directions, often crossing over each other on their way back.
Avoid placing it against walls or in corners. Both of these hinder the omnidirectional bass waves and will increase the possibility of bass carrying through the walls.
Avoid putting your subwoofer inside a cabinet. This is easily the worst place to put it and essentially voids any money you’ve spent on decent audio equipment.
Try to avoid putting the subwoofer under a table or other piece of furniture. It will cause gaps in the sound as the waves are absorbed by the dense mass.