You can use a pencil eraser to remove the discoloration that appears around the MacBook palm rest area, on either side of the touchpad. It takes a quite a bit of effort, and it definitely seems that plastic erasers are best (rather than India rubber), but it's an effective non-chemical method. Watch out for bits of the eraser getting beneath the keys.
[robg adds: I haven't tested this one, lacking a white MacBook...]

