Don't make mistakes more than once!
{jcomments on}Plans are like food - they have a limited shelf life!
This may seem obvious but is often forgotten. Nobody can predict/estimate what will happen for a very long period of time. Review progress on a regular basis and adjust your plans accordingly.
This does not have to be a cumbersome or tedious process, nor does it have to be time consuming. The key requirement for this is to adopt so called management stages to your projects from the get go.
A management stage of a project is a sub division of the project solely around management decisions.
It provides a control instrument for the project manager and the project board / steering committee to check progress of the project and to make informed decisions on how to proceed (continue, alter course, stop to name a few of the most common options).
Create, Update and Manage Your Scope Statement
Creating, updating and formalizing changes to the scope statement are important in meeting cost and time targets as well as satisfying clients. These practices likely reduce scope creep and keep everyone aware of any changes.
Do this by creating a formal change control process. This process does not have to be very complex. Key requirement for such a process is that there is some sort of official sign off by the person approving the change(s). Somehow people get more involved when they have to sign a physical piece of paper! 
Decsion Request for a simple decision request form
Source:Do project managers practice what they preach, and does it matter to project success? International Journal of Project Management
Karen E. Papke-Shields *, Catherine Beise, Jing Quan
Information Systems and Decision Sciences Department, Frankin P. Perdue School of Business, Salisbury University, Salisbury, MD, USA