I've been asked a question on twitter about seasoning a cast iron griddle- easier to answer here in full than in little tweets! There are lots of different views on the type of fat that should be used and the temperature that the heating should be done at. Generally though the below method is regarded as a good way to season your pans.
If you correctly season cast iron it will literally last you a lifetime. Being by preheating the oven to 180 degrees C.
Then you need to clean the pan carefully with soap and water. This is still important even if it is brand new since it will probably have a film on it of oil that the manufacturer put on to try and make it non-stick and you need to remove this.
You must then very very carefully dry it- for the seasoning to properly work the oil needs to soak into the metal and it won't do this if it is still wet (think of those school chemistry experiments where you try and mix water and oil!) When you have dried it put it into the preheated oven for a few minutes so that it is completely bone dry.
This is where you use the fat- both lard or bacon grease work well. Do not use butter or margarine nor vegetable oil which will just leave a sticky surface rather than seasoning the pan. The pan needs to be completely covered by the fat. If the pan has a lid this also need to be covered in the fat in the same way.
Put the pan (and lid) in the oven for 15 minutes. Remove after this time and pour off the excess fat that has accumulated (if there is too much fat it will go gummy rather than season the pan). Then put it back into the oven and bake for another two hours.
The more times you repeat this process the better the seasoning of the pan. When you start using the pan try and cook high-fat foods in it such as bacon and sausages since the fat will help with strengthening the seasoning.
If if the seasoning gets damaged or broken off in parts or rust appears then you need to repeat the above to reseason the pan.
Never keep food in a seasoned pan since it will give the food an odd taste and the acid from the food will break down the seasoning in the pan. Also don't using scouring pads or washing up liquid to wash the pans- clean while still hot with hot water. Otherwise you can damage the seasoned surface.
The pans will blacken as you use them and this is normal and nothing to worry about.
If correctly seasoned and looked after these pans will last far far longer than the modern day non-stick variety.