Retrospectives are incredibly powerful tools if they are used wisely. (Not unlike the Jedi's Force, it can be for good or darkness.)
One of my first suggestions on making them more effective, is to hold them more often. Holding them only at the end of the project means you can't apply any of the lessons to the project. Holding Retrospectives after major milestones, phase gates, etc. can be quick and quickly applied. I highly recommend the Manager Tools Hotwash podcast for an excellent and quick Retrospective for during a project. I marry this with the Scrum guideline of focus on fixing just one thing at a time.
Now instead of saving up a years worth of lessons, you can put learning into practice.
Retrospectives can still be very valuable after a project has ended. Reach into the PMBoK and it lists Lessons Learned as an Organizational Process Asset. During the chartering or forming of a project you would specifically look back on past projects for what can be learned from them. PM consultant Ainsley Nies used to specialize in Retrospectives. This was until she discovered that something like 80% of all Retrospective found issues could be traced back to the Chartering stage of the project. Now she is helping companies kick of their projects right, by looking back.