This is quick and simple. I don't know where I learned this trick, but it works so well I had to share it. It makes cleaning and treating your cast iron cookware simple without risking rust or removing the seasoning from the pan.
I start by wiping the big chunks out of the pan by hand. Then I use a board scraper (or a putty knife (I keep one with my camping gear just for this purpose)) to scrape the cooked on gunk out of the bottom and level the surface.
Then I pour about one tablespoon of kosher salt into the center of the pan. It acts as an abrasive.
I then dump some cooking oil into the salt, about one tablespoon. This is the lubricant that the abrasive (salt) moves around in.
The next step is to, using 3 or 4 paper towels (one at a time) go in circles around the pan from the center to the outside, rubbing hard on the bottom of the pan loosening the fried on gunk. Repeat as needed (usually takes 3 to 4 times) removing some of the salt each time.
Finally wipe the last of the salt out of the pan. Then use that lightly oiled paper tower to wipe down the outside of the pan to treat it as well.
You will be left with a perfectly coated cooking surface ready for use or storage. If you happen to miss some of the salt, that's no problem unless you are storing the cookware for 3 or more months.