Challenge long established assumptions. For example, application developers tend to specify new equipment to support their software.
However, an examination of the actual, rather than perceived, load may reveal a processor utilisation of around 10-20%.
If this is the case, consider combining non-critical applications on a single server. There is a major opportunity with test/development servers which could be switched off when they are not in use.
Typically this is not the case.
A few well executed actions are worth more than any number of well-intentioned plans. Involve your people, develop a strategy with them, and get started with the simple stuff.
The greening of the data centre can start now by implementing a few easily achievable actions - encourage your staff and your management along this road with you and the impact could be very surprising.