At the end of a project it is important to look back, especially to learn what things went well and which did not. This is not a matter of just the Project Manager but also of all those involved. I find it is critical to share knowledge and experience to help prevent the same mistakes again. For example, it is also crucial to share what has gone well because we can learn from others here too.
The sharing of knowledge and experience, both through presentations and storing in a database can help too. There are special 'tools' available in the market.
Think of sharing not only to 'hard' facts but also look at the 'soft' aspects. For example: how was the collaboration among team members, how were conflicts resolved, how was the atmosphere in the team, and was good communication kept both within the project team and outside with people?