I've heard a pool professional recommend to turn the pump on for a second. Obviously, the belt that holds the top housing to the bottom needs to be removed. Make sure to let all of the air out through the pressure release on the filter, otherwise the top housing could jump off and hit you (it's not likely, though).
Before doing this, take the lid off of your pump basket to relieve more pressure, and see if the top housing of the filter will come off then.