At Pivotal, I learned that having sprints is silly. They have the concept of one week iterations instead. The PM just keeps a rolling backlog of stories and devs just pick the next one off the top. Dev's are responsible for pointing the stories and the sum of those points averaged over time (usually 2-3 weeks) become velocity.
Just keep the velocity as consistent as possible and things work out for the best. If the PM wants a story moved up the list, then they can see how much work will fit into an iteration and how it will affect schedules.
All of this is managed in their homegrown, but publicly available tool called Pivotal Tracker. You can read the website documentation for a more thorough explanation. Once I learned this methodology and started practicing it like a religion, deadlines went away and PM's could estimate due dates many weeks in advance. It was a life changing experience that I highly recommend at least giving a shot at your own workplace.
Just keep the velocity as consistent as possible and things work out for the best. If the PM wants a story moved up the list, then they can see how much work will fit into an iteration and how it will affect schedules.
All of this is managed in their homegrown, but publicly available tool called Pivotal Tracker. You can read the website documentation for a more thorough explanation. Once I learned this methodology and started practicing it like a religion, deadlines went away and PM's could estimate due dates many weeks in advance. It was a life changing experience that I highly recommend at least giving a shot at your own workplace.