How to record and maintain lessons learned?Any methods please suggest

Usually there are templates for lessons learnt in every organization. The template contains, what was the issue, how and what it impacted, what you did to solve it, and what lesson you learnt to prevent such issues in future..

The best appraoch is to define the lessons learned detailes within the SOPs, for the organization. Sample, Project post mortem (aims at getting the lessons learned document done by the whole team), include all of the team, sahre the final file wiht the team and then store it within the project file.