Once every year or 2 years, try to remove all the keys from you computer keyboard and vacuum/wipe the dirty stuff that accumulated underneath. Then wipe the keys clean and put them back. The keyboard frame where you rest your wrists need to be cleaned of the sweat stains. It is a thin patina - when you rub it with a wet cloth, the dirt is visible.
Before you remove the keys, photo the keyboard layout or make a simple handwritten pencil copy of the layout. It will help to identify where to replace the keys after cleaning.