This is a very simple, cheap and effective way to fix your headphones if the plastic has broken or cracked and needs to be reattached or reinforced.
You will need:
Repair Putty aka Mighty putty - I got mine at Dollarama for 2.00
Small Phillips Screwdriver - not sure what size...
Black paint - latex, whatever
Unfortunately I didn't get the idea to make this repair an Instructable until half way through the process so you'll have to make do with the pics I took.
Step 1: Isolating the broken piece.
Unscrew the screw holding which attaches the bridge of the headphones to the earphone. You need to pop off the plastic casing and keep that and the screw aside in a safe place. Remove the earphone.
Next you need to detach the actual cup from the broken piece (looks like an inverted Y).
See the pictures for explanation.