Almost everywhere I look up information about caring for cast-iron pans, people are hollering at you not to put soap in your pans. They say in the most definite terms that this is extremely undesirable for your pans and will ruin the seasoning. This just isn’t true–I wash my pans with soap after every use, like my mother before me, and our pans are in perfect condition. Dish soap does not destroy the seasoning–it merely removes the layer of grease that you just cooked in, which is the point of washing something to begin with. As long as you oil the pan after every washing, at least for the first few months after the initial seasoning, you will build up a fine layer of season and your pan will last you indefinitely. You should see the way wash-water rolls off my most frequently used pan–the seasoning is almost waterproof at this point.
How do you get it seasoned like that in the first place? Easy: wipe it with a thin layer of lard or shortening (I use lard; I tried liquid vegetable oil the first time and it gummed up and I had to start over) and put it in the oven for an hour. Some people recommend an extremely high oven temp for this (450-500); others say 350 is fine. Both will work, but the key is a thin layer of grease–if the grease pools it will harden into a stubborn little nodule on your pan. Check the pan 20 minutes into the process and again at 40 (these times are for 350 degrees; if you use higher heat, check at shorter intervals), and if there are grease beads standing on it, wipe them away with a paper towel. Then, each time you cook in the pan, wash and thoroughly dry, then set the pan on a burner to heat for a couple minutes, put more lard or shortening in, wipe it all over the pan (again, thinly) and let the pan sit on the burner a couple more minutes, until the grease is very hot and well soaked into the pan. Turn the burner off, wipe pan with paper towel, and let it sit until cool. It’s okay if the pan remains slightly greasy to the touch.
For especially crusty, old, or rusty pans (or to clear off a botched seasoning job): I cleaned all the gunk of the ages off all my heirloom pans by putting them in the oven during a cleaning cycle–put the pans in the oven while cold, then turn on the cleaning cycle and leave them alone until many hours after the cycle is over, so that they cool gradually. Warning–some people say their pans have warped or cracked during this process, but mine withstood the heat and came out clean as a whistle–well, clean under the flaky ashy stuff, the remains of the formerly crusted-on stuff. From there, just wash, dry, and season. If they’re rusty, take some fine-grain sandpaper or a sanding sponge, sand on them for a bit, rub them with your seasoning medium, then wash with soap and dry thoroughly. Repeat sanding, oiling, and washing until rust-free. Then follow seasoning procedure outlined above.
This is what has worked like a charm for my pans–your mileage may vary.