fry bacon in it, or get bacon drippings nad smear a very very thin layer, and let it sit over a low flame for an hour or so to burn in
also you shouldnt use a steel wool, steel will rust, and when you scrub you leave bits of steel
i use those 2 dollar bamboo wok brushes from the chinese supermarket, and when the cast iron is still crazy hot, throw some water on it, and scrub lightly, everything usually comes off easily