Here are some tips:
Recycle (not only bottles and cans, but also paper and cardboard)
Compost (anything non-dairy, like veggies and fruits; you will waste less food, produce less trash and add nutrients to your garden)
Ride your bike if you can (to a local market, park, or post office)
Grow a vegetable garden (the closer to home the food, the less pollution produced and the more nutritious [no pesticides, etc.])
Buy clothes/linens made out of bamboo (bamboo is a very rapid-growing plant and therefore the supply is rarely low and quickly replenished)
Buy produce and other products grown/made locally (to save energy and any pollution caused by transportation of items)
Go vegetarian (if you eat lower on the food chain [plants, grains; etc.] you save energy [hard to explain, but basically if you eat an animal that eats the plants and grains, more energy is lost. If you just eat the plants and grains directly, less energy is wasted...hard to explain though])
Reuse the plastic and paper bags you get at the market (use them as small trash can liners or to transport bulky items)
Better yet, use reusable canvas bags when you shop (as to avoid using plastic or paper)
Unplug your laptop, cell phone, iPod and any other chargers from the outlet when not in use (believe it or not, energy is still used when not in use)
Replace lightbulbs with flourescant bulbs (regular light bulbs only use about 10% of the energy it takes up to light a room) (Plus they last way longer than regular bulbs)
More drastic measures would be:
Assemble solar panels on your house as to use more power/electricity from the sun.
Buy a fuel efficient car (not necessarily a battery-opperated car, because when the car doesn't work anymore, where will the giant battery go? In the ground? There are some negative effects of this.)
Take your name off of the list of recipients of junk mail
Buy energy-efficient appliances (refridgerator, freezer, washer, dryer, etc.)
If I think of any more, I'll update this answer, so check for updates!