Good: Print and copy on both sides of the paper.
Better: Use tree-free paper and recycle.
Best: Go as paperless as possible: send e-mails with attachments, write notes on a white board or in a computer program.
Bonus: Shred waste material for packing.
Good: Use the energy saver settings on your computer and printer.
Better: Turn off your computer at the end of the day.
Best: Unplug your computer (and other electronics) to keep them from leeching "phantom" energy -- electricity they draw even while turned off.
Good: Turn off all lights when you leave the room.
Better: Replace your lightbulbs with compact fluorescents or LED lights.
Best: Paint the walls of your office a light color to take advantage of natural light, preventing you from turning on the lights in the first place.
The Little Things
Good: Reuse paper clips or at least recycle them, otherwise all the extra metal goes straight to the landfill.
Better: Don't use rubber bands anymore -- they're made from crude oil and have serious health effects when they're incinerated at the landfill.
Best: Refill ink cartridges rather than buying new ones.