Season it when you get it.Even pre-seasoned cast iron can do with some extra protection. To season your pan, heat it up on the stovetop until its smoking hot, then rub a little oil into it and let it cool. Repeat this process a few times and you're good to go.
Clean it after each use. Clean your pan thoroughly after each use by washing it with soap and water and scrubbing out any gunk or debris from the bottom. I use the scrubby side of a sponge for this.
Re-season it. Rinse out any excess soap with water, then place the skillet over a burner set to high heat. When most of the water inside the skillet has dried out, add a half teaspoon of a neutral oil like vegetable, canola, flaxseed, or shortening. Rub it around with a paper towel. Continue heating the pan until it just starts to smoke then give it one more good rub. Let it cool and you're done.
Fry and Sear in it. The best way to keep your seasoning maintained? Just use your pan a lot! The more you fry, sear, or bake in it, the better that seasoning will become.
Don't let it stay wet. Water is the natural enemy of iron and letting even a drop of water sit in your pan when you put it away can lead to a rust spot. Not the end of the world, but rust will require a little scrubbing and reseasoning. I always dry out my pan with a paper towel and coat it with a tiny amount of oil before storage.