Remember, a lesson learned is not learned when you write it down for the 1st time but when you act upon it the next time!
In a lot of project management courses, books, manuals there is quite often a fair amount of attention on capturing lessons learned at the end of the project. A very important activity indeed. It is however only step one in a two step process. Not only that, it is not a static at the end-of-the-project activity but a dynamic project-life-cycle activity~
Step one: Write down lessons immediately when situation occurs. Do not wait until the end of the project!
Step two: Learn lesson by either not making same mistake or by doing what was recommended.
The crux of the matter is that step two happens not at the end of the project but when a similar situation arises. That might very well be during a new project but it is also very likely during the same project!
It is therefor important to have a Lessons Learned archive of some sort that is easily accessible throughout the life cycle of all your projects.