With an e61 put a blind basket in a portafilter, lock in, pull the lever to turn on the pump and just before the point that the portafilter gets locked in twist it out. It's like sticking your tongue on a cold pole in the winter; there's a time where you can pull it off but if you go past that time it's stuck. If you time unlocking the portafilter just before it's stuck the gasket and showerscreen get dislodged and they're easier to pull off.

See Removing shower head...