To save on paper, use disks to save work you've generated on your computer. If you're using your hard drive for this, that is fine too. Disks are good for backup though, in case your hard drive crashes.
Non-disposable cups
If your office provides disposable cups for beverages, eliminate the need for them by bringing in extra glasses and mugs from home. Post a note in the lunch/break room requesting that your coworkers do the same
Unoccupied rooms
Lights should be turned off in unoccupied conference rooms and offices. If you walk by an empty conference room, switch the light off. Turn lights off in your own office when you'll be gone for more that ten minutes.