Only print e-mails and files out if it's absolutely necessary.
Keep scrap paper: staple pages together to make rough notebooks, use the other side for writing notes or printing off unimportant documents, or even organize a scrap paper bin and make sure it gets collected and taken to a recycling plant.
Office paper all adds up: 70kg per employee per month! But by using recycled paper, in a year you'd save 12 trees, 15 000 liters of water (in energy terms, 720 liters of petrol) and a heck of a lot of waste. So petition your boss to buy recycled paper!
Always skiving at the water machine? Bring your own glass to use instead of a plastic cup you throw away every day.