I use hot water and a vegetable based soap to clean ours. For really stuck on stuff, soak the Kong in the sink and use a bottle brush (the kind people use for baby bottles) to get it squeaky clean quickly. My dog like to bury hers in the dirt after she's gotten all the good stuff out of the center. Because the Kong is made of a porous material, you want to stay away from cleaners with bleach or carcinogenicity ingredients as your dog will end up consuming them as they chew on the Kong.
I wouldn't recommend the dish washer though, because the soap used in a typical washer along with the intense heat from the washing and drying can cause the Kong to wear down, causing the toys integrity to be compromised.
Happy Play Time!