This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
For basic cleaning a little isopropyl (rubbing) alcohol on a Scott
towel works well. Common household cleaners like Formula 409 also do
a nice job. To perform a more thorough cleaning you'll need to take
the keyboard apart. Depending on the type of keyboard you'll need
one or more of a Phillips head screwdriver, a Torx T-15 screwdriver,
and a special tool almost impossible to find when you really really
need it which goes by the technical name of "key puller."
Disassemble the keyboard, pull off all the keys, and use a can of
compressed air or an electronics vac to clean out all the dust
bunnies that have mated and grown and had children and mated again
and built apartment complexes and shopping malls inside your
keyboard. (I don't know why, but keyboards attract far more detritus
than any other computer component.) Finally if you spilled Mountain
Dew, coffee or some other liquid substance into the keyboard, clean
it with lukewarm water and a soft towel. Use isopropyl alcohol on
any remaining sticky spots.