The trick is to scrub the line frequently while in the water using a deck brush and boat soap. That will slow down the buildup. To remove tough stains, you can try a hull cleaner like Starbrite On/Off. On land, you can try the hull cleaner, or for mild stains, a good cleaner wax. For tougher stains, you will need a polishing compound and elbow grease. For really tough stains, you may need a rubbing compound and a buffer.
For extreme stains, wet sanding may be necessary and in some cases, you just can't get the stain out if it penetrates the fiberglass it elf.