Last week, I have had two requests to help organizations implement Agile - but not greenfield, but overlaid on top or within Waterfall. What I discovered in common to both organizations is that there is a myth that this can't be done, or is nearly impossible, or even it CAN'T be done.
In fact, many organizations, if not most retain some kind of overall management approval process that provides a framework for Agile development. There are four 'next practices' that can help Waterfall and Agile coexist: Boundary Conditions, Attitude Influence Maps, Project Histories (a better way of doing retrospectives), and Demos. Two of these are what I would call SUPERPOWERS: Boundary Conditions & Prototype Demonstrations.
One best practice that enables this is the use of Boundary Conditions. This is a contract between management and the team - enabling trust for managers, and a free operating space for the teams. This boundary condition contract is declared early in development, and as long as the team is within the boundaries (schedule, cost, features, quality, etc.) they are free to develop without constant status reports. A win-win for both parties.
The second best practice is Demoing of Prototypes to an expanded view of 'customers' on a frequent (2-4 week) basis. Seeming trivial, Demos provide a way to get customer feedback. And you don't have to ship a product, or be doing web development. There are a number of ways to do Demos - even for Hardware. Furthermore, you don't have to Demo to real customers. Really meaningful feedback can be obtained by managers in your current organizations - but from different functions outside of your team's direct management.