Keeping your tires inflated to the proper pressure can improve your gas mileage by up to 3.3 percent. It's also safer, and will make your tires last longer (which ultimately results in less waste).
If you plan to drink your coffee at Starbucks, tell the barista that it's "for here." They'll put it in a mug for you rather than a paper cup.
Try using your eco-friendly dishwashing soap to wash your car since the runoff goes right into the ground. I tried it the other day to wash off all the sand and salt from winter, and it worked great.
I read the other day that a single printing of the New York Times uses about 70,000 trees. Time to go digital if you haven't already. Get your news online, use your phone, or try an Amazon Kindle.
Don't dump the remainder of your glass of water down the sink. Energy went in to purifying it and getting it to your house. Instead, pour it in your dog's water dish, or your watering can for your plants. They won't mind the backwash.
Don't use screen savers. Turn your monitor off instead, or configure your computer to automatically turn it off when not in use. Screen savers may save pixels, but they waste energy. Turning your monitor off saves both pixels and energy.
Turn your computers and monitors off at night, or between uses. It's not true that you'll damage your computer by rebooting it, but it is true that you you'll save a significant amount of energy.
Don't warm up your car in the winter. It's a fallacy that cars need to be warmed up in cold weather. As long as they don't stall when put into gear, they are warm enough. Warming up your car is for your own sake, not the engine's, and it just burns gas unnecessarily. Wear a heavier coat and gloves, instead.
If you rinse your dishes before putting them in the dishwasher (like I do), try rinsing them and stacking them on the counter as you rinse them, then loading them all in the dishwasher at once when you're done. That means you're not wasting water while trying to arrange and rearrange dishes to make them all fit.
Boycott styrofoam. Styrofoam (polystyrene) takes about 50 years to decompose, and is not easily recycled (they won't even take it where I live). It's pretty shocking that restaurants and places like Omaha Steaks still use it. Let restaurant managers know that the use of styrofoam is unacceptable.