The author of Social Media for Project Managers, Elizabeth Harrin, is a strong believer in documenting lessons learned.
Lessons learned capture what went right and wrong with a project. These documents are invaluable for future project planning and smart organizations adhere to this Project Management Institute best practice.
Harrin's blog creatively tackled the issue of sharing lessons learned in this post, Five ways for project managers to share lessons learned.
She recommends setting up a wiki to archive lessons learned and stresses the importance of READING the lessons learned to avoid "making the same mistakes again."
What exactly should be included in a lessons learned document?
"To be as valuable as possible, lessons learned should cover three areas: Technical aspects of the project ... Project management ... [and] Management ... ," according to Rita Mulcahy's PMP® Exam Prep, 8th edition.
A critical element of lessons learned is improving future projects. You may find value in this blog post, Getting Real with Lessons Learned by Bernadine Douglas, PMP, on the Voices on Project Management blog.