How do you clean the laser in a CD player?

Compressed air like Paulwp said. I would stay away from cleaning discs.

MCM Electronics sells a pen specifically made to clean the laser's lense:
SIMA lense cleaning pen

I agree with Bigwavedave. I have done this many times, and it always works, but be very carefull not to use to much force since the the laser would be dislocated and wouldn't read your cd's anymore. Compressed air is safer, but will only remove dust, and if you're a smoker the tar will pollute the laser.... so there's another reason to quit smoking ! :)

Go to Radio Shack & buy their CD lens cleaning disc. It has carbon fibre brushes on the underside that cleans the laser lens.
As an addition, RS has put a bunch of test tracks on that CD that you can use to setup your system.
Lens cleaner CD is $20 or so, if memory serves me correctly.
FWIW.