I use a product called Alumnabrite. It's so easy, just spray it on and immediatly hose down. It was so bright and shiney, just like new. A bottle of it cost about $30 and I did my trailer 3 or 4 times over the course of 2 years. I bought it at a marina store - it's also used on boats.
I think I went online and searched the product and there was a list of where it was sold.