Finding the magic in your magic numbers

Today's tip comes from a post I read by Scott Berkun:
The simplest, sanest step in the world, a step few people do, is when a project ends go back and review your estimates and compare against the reality. Put it up on a big whiteboard, sit down with a handful of your team leaders, and discuss two things: what factors explain the difference, and what smarter things you might have done to have a better schedule.

That's an excerpt of his recent post on Magic Numbers of Project Management. Read the post for full details on the tip, but follow the advice. I suspect you're asked to estimate your work on a regular basis, and understanding where the magic comes from is an important part of that task.