Corncob "media" from the pet store. I use it until it looks nasty, it will slowly take longer to clean over time. At least a dozen full tumbles before it needs replacing. I use a small amount of polish from Cabela's, nothing special, and they are clean in 2-3 hours. I forgot them for 24 hours once and they were blindingly bright!

If overnight is not getting them clean, try a different corn cob media, different polish or more of it.