I don't think you really can. Just need to make sure you clean your keyboard/mouse often. I have OCD and wash my hands probably 30-50 times a day, just can't get away from it. There is always something on your hands, weather it be stuff from food/drinks or just your own natural oils.
Nothing you can do short of wearing surgical gloves when you work on the computer. Which you would keep in a sealed container before using your computer. Better yet, do not worry about it and when they get too dirty, clean them. Same goes for dust which gets inside your computer. Eventually you need to open it up and clean it out. How often depends on how dusty your computer environment is and how much your computer is on.
you mean lick it off OFTEN! Clean frequently to avoid the crap, you can not really stop it happening. The longer you leave it the harder it gets, same for dust in your comp.
Well you'll probably slow down the process by washing hands often.
But some people are just naturally filthy. I bought a mouse off one such people, the amount of grime and general nastyness I had to clean was incredible.