Don't worry, it will outlast you.
I built a natural stone pit around a rusty cast iron grill for my father when I was sixteen years old, and that was when I was hoping there'd be a war in Viet Nam so I could be just like the guys on the "Combat" tv show.
I cooked a big barbeque chicken dinner for him and all his friends on that grill a couple of years ago for his 80th birthday.
If you're talking about the grate the fire sits on. Nothing. There is no trick or anything you can do. No matter how much you sand or steel wool or season with lard, or oil, or witches brew, it will always be rusty. Can't take the heat. That's why it got out of the kitchen and is sitting out there waiting for you to build another hot fire on it. Better hurry! If you take good care of your health and are very young, it might rust through before you die!