Baking soda will remove the polymerized oil. What type of oil you use is important. Animal fats don't polymerize like most vegetable oils do. If your pan smells like the last thing you cooked wash it, dry it with heat, and wipe a thin layer of oil to store it.

A metal spatula is necessary as well. Plastic and rubber aren't stiff enough to scrape the cook surface smooth.

I also use salt as an abrasive for stubborn bits.