Put it in a dishwasher. Seriously.

Or if you can't stomach that, dismantle it and wash the upper shell, lower shell, and all the keycaps (which pop off if pulled hard while kept perpendicular, though watch for the levelling rods under the space bar and the other big keys) in your sink using hot water and normal washing-up liquid. It's terribly therapeutic. I did that to a 10+-year-old SGI Indigo keyboard, and it came out good as new (though I should really have checked before wasting any time on it whether Linux would run on the thing).

Peter