Position - make sure nothing is blocking the fans, and if possible have the fans facing a clear area, away from walls. You can also try to raise it a bit from the ground, so most of the bottom part is not touching anything - you can use anything to create 4 "legs" to elevate it on.
Dust - If your PS is covered with dust, especially the fans, it would cause overheating. Clean it using canned compressed air (You can buy it cheap at any electronics shop) and a dry rag.
Environment - If you happen to be in a very warm and unventilated room, or live in a warm country, it would contribute to the PS' heating.
And lastly, if you are, then don't leave it working all day long.