I have had good success with a mild cleaner (soapy water or sprayway window cleaner) and a Mr. Clean magic eraser. Got off crayon and some other mystery stains. You might test that in an area that is not visible first just to be sure it doesn't do anything to the color of the leather.
Be careful with the Magic Eraser! It's an abrasive and is taking off the coating on the leather!
Jean stains are hard to remove however it can be done. I've used leatherique on my leather and it's gotten those jean and belt stains out easily. The longer they stay there, the harder it is to remove!