Hi there.  Since the subject of depressurizing portafilters for many espresso machines is all over the internet with varying degrees of detail, I made a video showing how to do this in a non-destructive, reversible way for the De’Longhi EC-155  Enjoy

The La Pavoni basket mentioned in the video is this one which is currently out of stock as of 4/25/2014 at both Seattle Coffee Gear and Orphan Espresso.  So, good luck finding that if you want it.