Now that we all agree that maintaining a sketchbook, and sketching and drawing on a regular basis, is important to the development of drawing skills there's something to keep in mind if you want your sketchbooks to last a lifetime. Of course, when you begin to sketch you may think that as the sketchbook only contains rough sketches and drawings, it makes no sense to go the extra mile and purchase a sketchbook containing good paper. By good paper I mean acid free, or neutral PH paper. At present I'm digitally scanning drawings and sketches made 35 years ago, and am finding that many have faded and some are in danger of falling apart simply because I didn't go the extra mile, and purchase acid free drawing papers. You may think as I once did that your sketches aren't important enough to last a lifetime, but consider that you might just become famous, or as I'm presently doing, you might wish to make use of them to archive your time spent making art. No one knows what the future will bring. Best, perhaps, to be safe than sorry years from now.