George and Bob Payne discuss the finer points of appropriately sizing user stories to ensure that they can be delivered. This is a fairly tricky process and is best figured out by experimentation and ensuring that you have good examples.

Developers must have good feedback to ensure productivity. Most shops have a continuous integration (CI) build process that allows the developer to quickly know if a build failed, and some shops have an automatic deploy of the CI build to provide website feedback. Lance Lyons writes about an approach to automating the update of databases in a CI environment.

In the past few months I've heard a couple of stories about (in effect) the disadvantages of experience when it comes to innovation and productivity. A Story on WBUR on July 5, 2011
discussed how venture capitalists tend to favor young entrepreneurs, as, having never learned the wrong things in business, they don't know what's possible or impossible. In one quote, a VC said:

Agile development practices at their core, have a common theme of making better use of the time spent developing software. This starts at the project level and continues down to the developer day-to-day-activities. Consider an agile iteration. The team starts the iteration with a clear sense of the priorities for the sprint, and pretty good understanding of the project scope. Having estimated and committed to getting the work done, the team also has a sense that the goal is attainable. The team members then collaborate to get the work done as a team.