Chris Kimball, editor of Cooks Illustrated magazine, was featured on Wisconsin Public Radio last July. He offered the listeners many excellent tips and advice, including the following process for seasoning cast iron:
#1: Throw away the instructions that came with your piece of iron -- they will NOT season your cast iron properly.
#2: Put your pan/pot/Dutch oven on the burner or in the oven on the highest heat setting possible for at least 20-30 minutes to burn off the factory coating and open up the pores in the iron.
#3: Put a small quantity of good-quality cooking oil in the pan. It will smoke like crazy -- that's good.
#4. Using a wadded-up paper towel, (and being careful not to burn yourself) thoroughly spread the oil all around the bottom and sides of your iron.
#5. Put the iron back on the heat for a while, then repeat steps two through four.
That's it! Kimball says you can tell a well-seasoned piece of iron because it is black and shiny. He recommends pre-heating the iron for a few minutes before each use, and immediately after you are done using the pan you should scrape it out and rub some oil in with a paper towel while the iron is still hot. Never use soap or abrasives on your iron. If it gets grungy enough that it needs more than a quick scrape-out, use hot water only (no soap of any kind and do not scour with an abrasive pad, steel wool or a soap pad). You don't have to worry about it collecting germs, as any that might take up residence will be thoroughly killed-off by the heat each time you use your iron.
REMEMBER: a shiny piece of iron is a well-seasoned piece of iron!