Toothpaste. But a more careful method (and I'm not talking **** here) is to take a green permanent marker and make the edges (around the center and around the whole disc) of your disc green.
It's very common amongst audiophiles to colour your discs that way. Somehow this causes the CD player to read less errors. For those of you who don't know; A CD player makes loads and loads of mistakes but it 'makes up' bits of music. If it can't make up music anymore, you'll hear a sharp 'tic!' or the CD will skip.
Anyway, when you have a perfectly good CD and colour the edges green, the CD will indeed sound better. Not on your laptop or on a all-in-one-stereo-set, but if you're pretty serious into audio, you will indeed hear the difference.
Using this philosophy, I once coloured the edges of a damaged Windows 2000 installation CD. And there you go, the PC could read the file.
Same goes with scratched CD's. It can just make that little bit of difference
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If that doesn't work; Toothpaste.