I like Hill's Violin Polish. A little goes a long way because you don't have to use it very often. For daily use just wipe it with a soft cloth (cloth diapers are marvelous for this b.t.w).

Back in high school my dad suggested saddle soap (an oil based cleaner) to clean the rosin/dust build up. I had quite the rosin build up- looked practically white. He took out his saddle soap from his boot polishing box and applied some of it on a soft suede shammy and rubbed gently in a circular motion like you would apply car polish, then with a clean shammy used the same gentle circular motion to remove the extra saddle soap and buffed it gently. It looked brand new! I have been using saddle soap ever since, but sparingly (every few months), which he suggested and it has never harmed my oil finish.