There are commercial kits available, but a can of compressed air, a lint-free cloth or wipes, mild dishwashing liquid, and a vacuum cleaner with a soft brush are all you really need for a basic cleaning.
After turning off your computer and unplugging your keyboard, gently shake the keyboard upside down over some newspaper to dislodge loose crumbs and particles.
Use the can of compressed air to blow a stream of air between the keys.
Dampen a lint-free cloth with a diluted solution of dishwashing liquid and water to wipe down the keys.
Use the vacuum cleaner brush attachment to suck away any remaining dirt or debris.
If you have a membrane-type keyboard (it doesn't have a spring under each key) and the keys are truly grungy, you can remove the keys with a small screwdriver and use cotton swabs and 90% isopropyl alcohol to get it sparkling clean. If you are still feeling industrious, next you can tackle cleaning your mouse.
Of course, if cleaning your keyboard seems a truly undesirable task, you can always buy a new one for around $20.