If your skillet is well seasoned, it won't be harmed by a LITTLE warm water and soap. On any surface (well seasoned or not) if there is heavy cleaning to be done heat the skillet a little and use oil and course Kosher salt with a paper towel to scour the areas needed. I have fashioned a scrubbing stick from a slice of a 1 x 6 about 1-1/4 inch wide. One end is cut at a 45 degree angle along the 1-1/4 inch plane. The other end can be shaped for comfort in your hand. Using this stick with the salt will keep the cooking surface clean and smooth for ever.

Soap will hurt the seasoning, but it won't hurt the pan. If you've got rust though, the seasoning is already ruined and you have to start from scratch, so go all out with steel wool and soap. Scrape the rust off with the steel wool, and then follow the seasoning tips from all the other people here.

We're in the no soap camp. Scrub with a paste of coarse salt and a little water. Oil it and put it in the oven. You will cook wonderous things in that pan.

As a former river guide, we always used cast iron dutch ovens and pans. To get rid of rust scrub with soap, as said before. Make sure to dry well and rub with oil. To clean we would scrub with sand (found on rivers) and water, then lightly clean with soap and water. As long as you don't scub to hard with soap you'll keep seasoning, unless of course your cast iron palette is finicky. Cook anything in it! It spreads heat evenly and almost makes cooking easy! Enjoy your great gift.

After years in a garage, I had some very rusty cast iron pans. As bad as they looked, they were restored quite easily, using all the wisdom above. I have others that I've used non-stop for years, with a permanent home on top of the stove. They are perfect for just about anything. I love the way that eggs and pancakes don't stick. It you saute something aromatic, the next thing you cook will release those scents -- but not the flavors so the next food is not tainted.

I use mine for pan grilling steaks, carmelizing onions, stirfrying veggies...

Anything that involves cooking with a bit of fat, I will do in a cast iron skillet. But I do avoid long simmers with tomato products as they tend to remove the seasoning in my experience.

I am with Pierino, I use soap and rinse. I find the seasoning is smoother in the long run that when not using soap. And I always driver over the burner or in a hot oven.

I remain steadfastly in the soap and rinse crowd and mine have lasted 20 some years. But you do have to rinse the soap off throroughly. Don't let it get anywhere near a Brillo pad though. In addition to cooking bacon in it I also use mine for deep frying. Bit of fried chicken definitely gonna hep' maintain that seasoning.

On the instructions that came with my skillet, it also said never to use soap?
I would love to get people's recommendations on what they use the skillet for? I hardly use mine other then for cornbread.

Never ever use soap.

For rust or stuck on food the people at Cook's Illustrated recommend putting a 1/4" of veggie oil into the pan, and heating it over medium-low heat for 5 minutes. Remove from heat and add 1/4 cup kosher salt. Use potholder to hold the handle and a thick pad of paper towels to scrub the pan. Rinse under hot running water, dry well, and repeat if necessary.

Always dry it on the stove or you'll get that rust coming back again...

Agree with all the above I also keep one needing care in the oven for a while and make sure it's lightly oiled. It gets a little extra seasoning every time I preheat the oven.

Another way to season is to oil it all over (after scrubbing as pierino says) and put it in a low oven for an hour or even more. Then use it! Regular use is the best way to develop the season. You won't want to use soap on it, except in the case of dire emergency. Once seasoned, you can wipe it clean and it'll be fine, most of the time. You can use water and scrub at stuck spots. Then dry it on the eye of a stove and rub the interior with oil again. Keep enthusiastic dishwashers who may want to "help" and don't know the value of that lovely black seasoning. I've had a well-seasoned pan completely de-seasoned that way. I started from scratch and got it back in order.

Scrub it in hot soapy water with a plastic scrubber. Rinse it and burn it dry on a gas burner. Rub it all over with canola or vegetable oil and stick it in a very hot oven for 20-30 minutes. Wait, you are not done yet. Cook some bacon in it. Repeat until black and seasoned. This could take weeks.