Have you ever heard the old quote by the philosopher and poet George Santayana that states, "Those who cannot remember the past are condemned to repeat it"? In project management, we need to remember that historical data is our best friend when planning new projects. Do not forget when doing your planning to use empirical data from past projects. This data can help you to reduce negative risk and increase your odds for project success.
Other information to review when planning new projects:
Review your companies past project files for information regarding past resource estimates, lessons learned, budget data, risks, assumptions, etc...
Conduct interviews with select project team members from past projects to understand what went right and what went wrong.
Interview customers and other project managers for lessons learned from their past projects.
Do searches on the Internet about similar projects to gather information which might assist in planning your project.
Most importantly, use Risk Management during the planning cycle to identify issues that could cost you big later on.
Finally, do not fall victim to the project manager's curse of not learning from the past. Remember the old saying, "Those who cannot remember the past are condemned to repeat it"