There is no fool proof method. Apply liberal amounts of heat. Be careful when slicing near the corners. If your back has micro-fractures already, chances are your back will crack along the fracture.
No matter how long I try to heat the back panel with an iOpener, I cannot seem to get the glue warm enough… Any advice on how to better unglue this thing?
The glue is pretty strong on this phone. You can try using a heat gun if you have one, or a hair dryer. You can also try the bottom edge instead of the long edge- sometimes the glue is not as strong there.