If they’re really bad, you can use very fine grade (grade 0000) wire wool (don’t be too enthusiastic with it though), followed up with a metal polish like Autosol. I’d always try a metal polish first though, before resorting to using wire wool.
The key to ensuring they don’t get really bad and go beyond the point of no return is frequent/regular cleaning. I find normal washing when I wash the car is usually sufficient, and if there are any stubborn marks that washing won’t remove, a small amount of medium / light cut polishing compound that I normally use with my DA polisher, or some Autosol will get rid of the stubborn marks.
