I tend to keep my cars for a long time, so I put a "Wheelskin" cover on my steering wheels as a sacrificial cover (intending to replace it when it gets ratty). http://www.wheelskins.com/
But I have been touching them up with spray leather dye I goy from a leather shoe repair shop. More recently, I've used vinyl dye out of a rattle can from AutoZone/Advance Auto with excellent results!
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