Ideally when stripping you end up with just the skin of the quill and no pith.
And having fresh feathers is ideal. When they have dried out for over a year or so, they are in need of a little "refreshment". Pat suggests a plastic bag and a damp towel for a day or so, I recommend something faster: A hot teakettle on the stove blowing a steady stream of live steam. Slowly draw the feather through the steam and watch the feather barbules plump up. Concentrate the steam on the central spine both on top and on the bottom of the feather.
After steaming, they will strip much, much cleaner and often with fewer of them breaking off prematurely.
Oh, and for those that believe that a split and ground feather is stronger than a stripped one without any pith, just remember that BOTH end up with the same stiffness - namely whatever spine the arrow shaft hasa after they are glued down!