Every time I wash my cast iron skillet (I don't use soap if I can help it.) I season it. And all that means is: once it's washed and dry, I rub oil all on in the inside of the pan and lip and place it in a warm oven. That's it!
If the oven is still warm from use, excellent! If not, I just warm it up to 150º F then turn it off and put the pan in. I just let the cast iron skillet pan stay in there until I use the pan or the oven next. (Remember to always look in the oven when you turn it on.)