It seems like most 360s are freezing due to overheating. My Xbox 360 was freezing every 20 minutes. When I put the PSU on this box I left the console on for 7 hours without crashing (I finally decided to turn it off).

NOTE: If your 360 is not freezing because of overheating, this fix WILL NOT work.

So I took the power supply off the ground and suspended it in the air with some string. I have been playing for about 3 hours straight and no crashes so far!

EDIT: I found an easier way to do this. I got a box with no cover and put the power supply on the edges. This is just one way of doing it. The main point of this fix is to get your PSU off the ground for maximum heat release.