The two most important steps in the development of Western European monasticism were the creation of the Rule of St. Benedict and the later reform of the Benedictine Order by the Cluniacs.
The Rule of St. Benedict was written sometime in the early 500s AD. It set up a common rule for how Christian monasteries would be set up and how they would conduct their daily affairs. This rule brought order to the Christian monastic movement. It made monasticism repsectable and it gave rise to a number of daughter monasteries that spread across Europe.
Some 400 years later, the Benedictine Rule had lost some of its purity -- Benedictine houses no longer acted with as much piety as they originally did. At this point, the Cluniacs stepped in and started a major reform movement meant to bring monasticism back to its original holy state. This movement helped to restore monasticism's past status in the Church.
Therefore, these were the two most important steps in the development of monasticism in Europe.