Once you have done a full postmortem on your project, you should document all the improvements and best practices gathered by your team. At the very least, you should write them down. A more effective way of retaining the value of the lessons learned is to modify your process templates, immediately transforming the recommendations into templates, policies, work items, and workflow. For example, you may want to include steps in your work item instance (the default work items that appear when you create a team project). You may want to enforce code policies (for example, making sure that all your classes and methods include documentation) and add best practices documentation to the project portal.
You can improve your process by studying the special cause variation on a project. A lot of the mechanics behind formal processes are designed for the purpose of accountability and conformance to specification - in other words, documenting, evaluating and auditing each step of the process to make sure the quality of the end product remains high. In agile methodologies, trust is inherent in the process; you have to trust that your developers will build quality features and build trust with your customers. MSF for Agile Software Development and MSF for CMMI Process Improvement foster agility by allowing your team members to focus on their core activities, automating documentation and policies, and, most importantly, building quality within the process.
One of the vehicles for incorporating your best practices in your process is the Guidance Automation Toolkit. It allows you to compile your guidance and enact guidance through mechanisms such as code automation.
You can learn from your process by looking at the risks and issues that came up during the course of your project. You should query them, aggregate the data, and create an action plan for future projects.