You need to give them a good cleaning (APC, Griot's Rubber Cleaner, etc.), try with a MF towel (don't use one you're too attached to, it may wind up with permanent stains).
Then use a good trim protectant like Wolfgang Exterior Trim Sealant (WETS) or Ultima Tire and Trim Guard (UTTG) or Blackfire Total Trim & Tire Sealant (BTTTS?).