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]
Using Project Management Templates
At my last engagement, there was a lack of matured documented process. I'm all about process. If you have good process, quality results will follow. People sometimes don't understand that if a documented process does not provide value to the project, you should not do it. Consistently use proven templates, processes, and methodologies to refine your project management approach. Templates are mere tools in a toolbox. Again, only use the ones that provide value to the project. I once had a Product Manager voice her frustration because she was told to complete ALL of the project templates in the repository, after the project was in a later phase. My response was to weigh the cost (time) of completing the templates against the benefits gained by them. The benefits are traditionally lower risk or higher customer satisfaction. In this case, it was neither. If anything, there should have been Lessons Learned or Postmortem document that would have identified the gaps or overlaps of the documentation.
To assist readers of my blog, I'm going to start uploading templates I've used at previous engagements. They are free of change and will fall under public domain. You'll find them by selecting Free PM Templates or locating the link in the right navigation menu.