There are a few ways to look after your cookware, but for those folks really serious about maintaining their it, or who have money to spend on them, flax seed oil is highly recommended. It gives your cookware the ultimate non-stick, rust-protective seasoning, as well as a gorgeous shine.
The trouble with flax seed oil is that it can be pretty expensive and difficult to come by, especially if you don't have a health food store in your vicinity. If you do, however, Instructables user noahw claims it's the best way to season your cookware, and he has the shine to prove it. After doing some research into the topic, we turned up a number of people who agree, including an article in February 2011's issue of Cooks Illustrated and this Cheryl Canter piece that inspired it. To use this method, you'll need a few millilitres of flax seed oil, a 260C oven and a few hours to rub down the cookware with the oil and bake it in the oven. The process of seasoning is the same as using any other oil: give your cookware a light coating on all sides with the oil, bake it in a hot oven for a half-hour, remove the cookware, re-coat it with oil, return it to the oven, and repeat the process several times (noahw suggests 4-7 times.) The use of flax seed oil reportedly offers additional protection and shine.
The idea isn't without controversy: there are just as many articles on the web defending the practice as there are refuting it, including this thread at Reddit and this one at CHOW debating the idea. So your mileage may vary, but if you're looking for a new or better way to season your cast iron, this method may be worth a try.
Best Way to Season Cast Iron Pans - Flax Seed Oil [Instructables]