If your cast iron grill is new, wash it with warm water and a gentle dish detergent. Rinse and dry it completely. Coat the surface of the grill with a thin layer of your preferred cooking oil. Bake the pan upside down for an hour in a 375 degree F oven to bake the oil into the pores of the grill pan. After baking for an hour, turn the oven off and leave the pan inside until it cools completely.
Coat the grill with a thin layer of cooking oil or butter before you heat it, then slowly bring it to temperature over the lowest possible heat for your purpose. Add food to the grill at room temperature, as the drastic temperature difference between cold food and a hot surface may cause food to stick to the grill. If food sticks slightly, wait for it to release easily before you turn it. If your cast iron is properly seasoned, the food will release when the side on the heat is seared properly.
Avoid using soap or other cleaning products on a cast iron cooking surface unless it is absolutely necessary, as the soap will strip the seasoning from the pan. Repeat the seasoning process after the grill pan is dry if you do use soap to clean it. Place the grill under hot running water and clean it with a stiff-bristle brush to clean any debris from the surface. Dry the grill pan with a towel right away to reduce the risk of rust.
If your grill pan develops rust spots, it is not lost. Clean the rust away with steel wool or a similar abrasive pad. After you remove the rust, season the pan. The same process will prove successful if food starts to develop a metallic flavor, indicating that the seasoning has worn off some patches of the grill surface. Store the grill pan in the oven or similar area to keep the air circulating around it to reduce moisture.