How to clean a keyboard?
To clean out the crumbs, hold your keyboard upside down over a wastebasket and shake it gently. Then hold the keyboard vertically (still over the wastebasket) and use a can of compressed air to spray between the keys. These two actions should dislodge most of the physical grit.
Though plenty of computer-cleaning products are on the market, most are designed to remove only ordinary dust. To obliterate both germs and dirt, antibacterial wipes such as Wet Ones Antibacterial Moist Towelettes should do the trick. Make sure your computer is off, then give the surface of the keys a once-over with the cloth.
While you're at it, give your mouse a thorough wipe-down--it can collect plenty of dirt and finger grime, too.