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.

Step 2: Attaching With Putting

Open up your putty stick, slice off a small piece (like the size of a grape). Knead it until it's a uniform colour.

When doing this part, make sure you avoid getting putty in the area where the plastic has to snap back in and be screwed on.

You need to work quick as the stuff starts to harden about 3-5 minutes.

Step 3: Finishing Touches

After you've applied the putty and waited for it to harden you may have to use a file or sandpaper to grind it down in certain spots.

Once you've done that, you can add some paint to make it blend in.

All that remains is to snap the piece back on the ear cup and then reattached the whole thing to the headphones... Voila good as new.

Check out the power of mighty putty here.