I was recently reading a post on InfoQ, "Frequent Retrospectives Accelerate Learning and Improvement". As it's title suggests, the key messages in the article is about having frequent retrospectives to aid in the learning and improvement process.
When we adopted XP (eXtreme Programming) we undertook to have a retrospective at the beginning of each development iteration, preceding the planning game. With weekly iterations, we have a chance to reflect on the previous weeks pluses and to formulate some changes/improvements (deltas) identified from the week.