What type of sketch book do you use? I have a hardcover sketch book which prevents the pages from shifting, but the graphite will still smear while I'm working on it. I don't have any experience with a spiral bound sketch book, but I can see the pages shifting often in one of those, causing the graphite to smear.
I usually use ballpoint pen to sketch: it's still possible to lightly shade areas in, it doesn't smear, I can't erase anything so my sketches are more spontaneous and raw—which is good for the design process, and it also trains me to be more confident with my marks and to be more precise. If not for the fact that pen doesn't smear, I'd suggest sketching with a pen just for the other reasons alone.
Another possible option is to use black Prismacolor pencils instead of graphite ones. Those won't smear either, but if you have an electric eraser you should be able to erase the marks. It may need to be on a tougher paper though. I heard about the Prismacolor trick from a story board artist and I can't remember what paper he used. So I'd try it out first.
Either way, it's a sketchbook. You shouldn't really be worrying about erasing anything.