put the phone in the microwave... easiest way...
.. i'm joking... please DON'T do that... lol..
i had the same prob with one of my covers, i left it in the pot tooo long (forgot about it and went on a date), anywho, i learned that if you press down a little harder on the bottom of the battery case (where the lanyard hole is) while pushing towards the top of the phone, it'll pop off.
this is a result of the battery case being somewhat deformed from the heat...
-u