My favourite method is to first paint the face of the CD/DVD lightly with with a non-corrosive glue (PVA is best), then liberally apply iron-filings, allow to dry.
Next insert into a bowl of washing up liquid and a little white spirit or alcohol (so the disc is just beneith the surface), then pop it into the microwave for maybe 30 seconds.
The best thing is too look throught the window and when you see the washing up liquid start to bubble; then you need to take it out and lightly smoothe the face with a soft piece of skin, preferably your cheek on your face.
Another good way that I've found is to coat the disk in polysteyrene cement, and then tapping it off veeery gently with a chisel.
Also, stretch cling film over disk, heat with hairdryer until the plastic has covered the mirror surface. Cut away the excess bits around the edge.
Write on a small white sticky label ' working ' and stick it on the top after doing all of these, and you're guaranteed success!