Here is what I use:
For mild/moderate cleaning Simple Green works great.
For heavy yellowing I use Purple Power which removes a lot of, but not all,
of the yellowing (careful this can discolor metal).
I use both with the Black and Decker Versa Pac rotary scrub brush with the
soft brush attachment, if the case is removable, in the sink or driveway
with a hose.
Simple Green diluted 1:1 with water with a rag and Q-tips for cleaning the
keyboard and parts of the computer near elecrical components that can't be
hosed down or put in the sink as in step one.
Goo-Be-Gone for spot cleaning of residue from stickers etcetera.
Meguiar's Protectant as a last step.