I had some success with Isopropyle Alcohol (Guardian, $5 for 200ml) and some baby powder. If you don't mind the greyish look...
But different rubber types / mixtures will give different results, so replacing is the cleanest way.
Normally I would use a silicone product. http://en.wikipedia.org/wiki/Silicone_grease
For rubber o-rings of my underwater camera I would use silicone grease but it makes the rubber greasy. For something less greasy, I would use this: