When making short stops, of about 10 minutes, turn off the monitor screen, because it is the part of the computer that consumes more energy (between 70-80%). For stops of more than one hour it is recommended to turn off completely the computer.
Energy savings between 10 and 20% can be achieved with proper training
Avoid internal heat gains
Extend the lifespan of the monitor
The cost is zero, although it is convenient to train and inform properly to the users on these issues.
References and best practices
A case study on the individual energy use of personal computers in an office setting and assessment of various feedback types towards energy savings: