A professional leather cleaner can work miracles. The local tailor shop when I lived in Florida could take a dirty, sun cracked motorcycle jacket and make it look new. Around here, I might look to a saddle and tack shop for that sort of restoration work.

For a more workaday solution, I use Chamberlain's Leather Milk cleaner and conditioner to clean and treat my shop apron and other leather goods when they look particularly dirty and/or dessicated. Does not do magic, but does keep things clean and supple, and will buff out light scratches.