An excellent article on keeping your knives in order by Chad Ward.
Cast iron pans should be black due to a protective layer of oil coating the steel. If the pan starts to turn grey, you need to re-oil the pan. Evenly coat the pan with cooking oil and put in a 220’C oven (the oil should be just below smoking point) for half an hour. Turn oven off and leave pan inside as it cools.
Do not cook with lots of sugar or tomatoes for prolonged periods of time as it can affect the oil coating.
Do not leave food to stand in cast iron pans. Clean immediately after cooking, don’t soak for long periods of time. Dry after washing. wipe with a drop of oil if the surface looks dull before storing.
Scrub with steel wool and scouring powder, rinse and dry it. Heat for a minute or two until the bottom is too hot for your hand. Rub with a paper towel and oil and let it stand overnight. Sprinkle a teaspoon of salt in the pan, heat again, and rub vigorously for a moment with paper towels, rub pan clean.