Is the rubber damaged, or just turning a chalky grey?
From new, I have used Back to Black or Black Chrome. One is Meguiars, the other Mothers. Can't remember which is which without going to the garage to check.
I put it on a couple times a year, and my boards still look new. You could try one of those products first. It will likely take several applications to see some improvement, but start with a thorough scrubbing with a tire cleaning product to get the oxidation off.
If you don't see enough improvement with the products intended for black rubber exterior moldings, the next step would be to re-clean thoroughly to get rid of the product you just put on, and use a dye product for rubber.
The guys who remove the rubber and spray the step insert areas with a box liner product seem happy with the results. I prefer the smooth look of the factory rubber. It takes some scrubbing to keep the textured liner product clean, and probably some time removing the tape residue from the raw boards.
I like how Dicktator did his boards in the photo above, with three strips instead of a complete coverage.