I've never, ever, ever felt the need to wash a rucksack; the very idea seems alien to me. Perhaps try disinfectant to kill any bacteria, thoroughly rinse and dry it out, and then let nature take it's course. Or maybe I'm just a disgusting person!
And a good belt is a godsend on a larger pack, up to 70% of the weight transferred directly onto the hips rather than onto the shoulders and spine. I believe that the padded belt should sit over the top of the hip bone. Even on a small pack it helps to stabilise the pack if you are descending rough ground.