Most internal doors can be cleaned effectively with a mild solutions of soap and warm water - this is usually enough to lift every day marks and stains. A soft cloth should be used to avoid damage to the surface; making gentle passes over the marks in a circular motion.
For more stubborn scuffs and stains, you can use a diluted mixture of distilled white vinegar and water on most finishes. Tip: a solution of distilled vinegar and water at a ratio of 1:10 is great for cleaning the glass of glazed doors.