Let me start with NO soap, NO detergent, NO dishwashers, nada, nyet, nicht, nader, none…
The proper and ONLY way to effectively clean your iron cooking ware, (without ruining its seasoning), is with hot water and a a good stiff bristle brush. If you have some stubborn residue, simply put it back on the coals, (or burner), and let the hot water loosen things up.
After a good scrub down, then rinsing with more hot water, I like to heat the iron back up to make certain ALL of the water is gone, then treat it with a quick wipe down of cooking oil with a paper towel or two, or hit it with some cooking spray like Pam (don’t inhale the steam) and spreading that evenly with a paper towel. By removing all of the water and treating it with a little oil, you will retain/enhance your seasoning and your dutch oven, wok, frying pan, etc., will be ready to go next time.
I own one of these scrubbers:
It only gets used when I have run into the real stubborn food particles stuck to the iron, and find myself “boiling it loose” – but these days that doesn’t happen often unless I burn something. Generally I am able to just use a little hand brush and plenty of elbow grease to get the job done.
WARNING! NEVER put anything cold in a hot piece of cast iron cookware. Sooner or later you will crack it.