Are you talking about the Sanitary Commission type bags for provisions? I just rinse mine good with warm water, turn them inside out and let dry well before storing them back in my haversack.
They are very handy sacks. Main thing is make sure there's no moisture on the inside before you store them away or you'll start a science project. When I'm in a busy stretch of events I just turn them inside out after use and put them on the shelf that way till the next time.