The most obvious method of keeping the cooking grates on your grill from becoming rusted and terrible looking is to use them daily. This would be great for several of the most obvious reasons! However, in most circumstances, this is not practical.
The best method to retard/eliminate the grate from rusting would be to take the following steps after each cooking event and while the grill is still warm/hot:
1. Scrape the grate clean from any cooked on foods/products.
2. Rub down the grate with a cooking oil/lard.
3. Place the grill in a covered location until the next use.