Using a damp warm cloth has always worked just as well as any kind of soap, which can dry out the leather even more. If I do use soap, I use Murphy's Oil Soap, but always sparingly. You didn't ask this, but I really really love the conditioner Tatersall that you can buy through Dover. It's much better than any of the spray stuff or saddle soap/glycerine bars. It's gives a nice shine, conditions the leather, and doesn't make it slippery.