I've had great results with Armor All Original Protectant.

Make sure the knobs are clean. I use a soft toothbrush and a general
purpose cleaner like 409 or fantastik. Isopropyl alcohol works for very
dirty knobs. After the knobs are dry, I spray them with the Armor All and
let them set for a minute or two. Dry them off with a paper towel and they
look like new.