Try not eating directly over it, using a plate, covering it with a hankerchief, or using one of those desks with a slide out tray to put the keyboard/mouse. Some places sell keyboard covers though they only work for one specific keyboard each.
I try not to eat while I am on the computer. I have a a piece of plexi glass that I can see thru that extends over my keyboard though because I smoke and was dropping ashes on the keyboard. I suppose this would work for crumbs also.