It is not important that you call it a Lessons Learned or a Postmortem. What is important is you learn from your mistakes. At the end of every iteration or project, you should identify everything that went wrong or could have gone better. Articulate them in a document. At the beginning of each new iteration or project, review the document and see not only what was learned but what has been implemented. Feel free to download my free Lessons Learned [Template]