I have spent the last year working with PMI on creating a Practice Standard for Estimation as the lead author. The key premise of this Practice Standard is that estimation is a life cycle and not a one time activity. This is a critical concept to understand because a lot of times a project will do their initial estimate and then when the actuals come in much higher they will say that the estimation model was wrong. In actuality, the project may not have been run diligently and brought in costs that were not part of the original estimate.
As the picture below shows, the initial estimating is only the first step in this life cycle. Once that is complete it is critical that the Project Manager diligently run the project (step 2) and manage scope (step 3). These are critical to managing to the estimates to avoid unplanned costs. The last step in the cycle is to take actuals from the projects to calibrate the estimating models for more precise estimates int he future.
The keys to good estimating are diligence in creating the estimate and then managing to it. Project Managers can not be successful without both components.