The problem is that you have grounds accumulating up and around the gasket.
You could either:
a) purchae a group head cleaning brush and just gently scrub the gasket after each session
b) grab a damp cloth, bunch up a corner and insert it into the gap. Wipe around the gasket making sure to keep your hands off the shower screen (its hot!)
I personally have never bought a group head brush, so I just bunch up a damp cloth