I like Simichrome or 3M Marine Metal Restorer and Polish.
If the rust doesn't come off with a polish (it probably will), then you can start with something a little bit more abrasive and work your way back smoother (very fine wetsanding paper, a 3M pad, etc.).
Like Alf said, it's important to have as smooth a surface as possible. The more you keep it smooth and polished, the better it will stay shiny and non-rusty.