In my experience with existing buildings, if you follow the four T’s for saving energy: Tune it up, Turn it down, Turn it off and Track it, you should be able to reduce energy consumption by at least 10-15% without significant cost.
Tune it up would include optimizing your ventilation rates (you want to condition the least amount of outside air necessary to maintain proper air quality inside your building) and your conditioned air distribution system, calibrating your thermostats and sensors (because inaccuracies can cost energy), and performing periodic maintenance of all building systems including regular boiler tune-ups and filter changes, cleaning heating and condenser coils, and window and light fixtures.
Turn it down would include night setbacks to ensure lighting and HVAC shutdown during unoccupied periods, and reducing heating and cooling temperature controls (for example heating to 68-70 degrees F, and cooling to 74-76 degrees F).
Turn it off would include shutting down office equipment when not in use (e.g., computers, printers, monitors, coffee makers should all be turned off overnight and on weekends). Occupancy sensors, time clocks and automatic controls can easily accomplish this.
And finally, Track it would involve monitoring and recording daily energy use by category, comparing it to regional or national energy use benchmarks for similar buildings, and finally, making sure that an energy conservation ethic is maintained over time.
If you follow these simple four T's, you will get the greatest bang for your buck. All of these steps can be done relatively quickly and have an excellent ROI.