Drink tap water where it is safe. Bottled water is expensive and contributes to the epic waste problems the world has. Also, the fossil fuels used in manufacturing the bottles drains the available fuel supply and, in cases where coal or nuclear power is used, emits harmful gas into the atmosphere.
Brew your own coffee. Buying coffee all the time from Starbucks and other companies is a huge waste of money. ($3 a cup compared to $3 a pot?) Also, though companies like Starbucks do used paper sleeves and cups with post-consumer recycled content, the fact is that they are still using the paper.
Buy energy-efficient bulbs. They save you money in the long run, and also reduce energy usage.
Turn off your lights and appliances when not in use. This saves you money on your electric bill by reducing how much energy you use. The best way to do this is to buy those big surge protectors with all the plugs. You can just turn the entire thing off when you're done, and actually TURN OFF all the things plugged into it. Remember, when that little red light is glowing to say your TV isn't on, it is still using power!
Print on both sides of the paper! When you're printing things out for class, or even just for recreation, print on both sides of the paper instead of just one. Yeah, it might take you longer to feed each piece in individually, but it will reduce the money you spend on paper, and also help save trees.
Carpool, bike, or walk when you can. This saves money on gas, and also prevents more harmful gas emissions from going into the air. Also, biking and walking don't hurt your body at all--they actually help.
Use regular dishes and cloth napkins and towels. Paper plates, plastic silverware and cups, and paper napkins and towels all contribute to waste, and they cost more than just reusing regular dishes and cloths.
Recycle your pop bottles! You get your deposit back, and the environment gets a break.
Wash your laundry in cold water. Hot water uses more energy (electric or gas) and thereby hurts the environment. The clothes get just as clean; I promise!
Know when to use your windows. If you are driving less than about 70 mph, drive with the windows down instead of having the air on. If you are going over 70, do the opposite. The faster you drive, the more resistance your car experiences with the windows down, and the more gas you have to use. But if you are going slower, more gas is used when you use the air. Also, at home, turn on the air only when it is nearing uncomfortable temps. If it's 70-80 degrees, just open the windows! Fresh air is fantastic.