You must be clear on your rank order priorities at any point in time; but you must also be able to quickly incorporate new or changing requirements. I use the analogy of water and ice. Most of the time, your backlog is like ice; the rank order is frozen and fixed. But when new requirements come in or priorities change, you briefly melt the ice into liquid water so you can rearrange things. Once you're done reordering your backlog, you freeze it again. Following this approach means that your backlog will be up to date whenever anyone looks at it.
