I have a few pieces of cast iron, and I wipe them clean with a paper towel and some olive oil while they're still hot. If there is food stuck on (like bacon grease), I toss in a little water and the steam produced seems to loosen the hard bits. Hit it with a plastic egg flipper or nylon dish brush while the water boils. Then I do the oil treatment, and I'm done. I never use soap or detergent,ever.
My ex-wife put all of my cast iron in the dishwasher once...