You can find flax seed oil in the refrigerator aisle at the grocery or health food store. Flax seed oil is the edible version of linseed oil, a very durable, hard drying finish that painters and woodworkers have been using for a very long time. As Cheryl Canter writes on her site
: "The seasoning on cast iron is formed by fat polymerization, fat polymerization is maximized with a drying oil, and flaxseed oil is the only drying oil that’s edible."
What that translates to in practical terms is a durable finish, that even after just a few coats and short term heating results in a deep glassy black seasoning on the cast iron that has held up to months of my daily usage and cooking abuse.
As with any other cast iron pan seasoning, You don't want to use soap on the pan when cleaning it, but with this method, I've found that using a mildly abrasive sponge when doing the dishes doesn't seem to affect the finish at all.