Often with old tubs years of abrasive cleaners have ruined the porcelain finish. I know ours is like that. The "luster" is forever gone. Short of having it refinished, the best you can do is keep it clean.
I would start with vinegar. Fold a paper towel to cover the stains. Soak the paper towel with vinegar and press it on the stain. Keep it moist with vinegar for several hours at least. This may or may not do anything, but if you have the vinegar on hand, it doesn't cost anything to try.
Edited to add: A pic may help. This is a porcelain tub, right?
Last edited by Blondesense; 10-06-2011 at 12:11 PM.