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.
Don't throw away your tools' rechargeable batteries when they no longer take a charge. Take them to Home Depot where you can drop them off for recycling.
It's time to replace those incandescent bulbs with CFLs, or compact fluorescent lights. CFLs use 75% less energy than incandescent bulbs, and can last up to 10 times longer. They don't buzz or flicker, and most turn on immediately. The color and quality of the light may take a few days to get used to, but the savings are worth it.
Use power strips for things like TVs, A/V components, video game systems, etc. When not in use, turn off the power strip to cut power to everything. Even when turned off, things like TVs consume power, so using power strips to control several components at once makes turning them all off much easier.
When you receive a package with styrofoam peanuts, don't throw them away. Dump them into a garbage bag and save them for when you need to ship something. Of course, you can also reuse most boxes you get, as well. This is probably one of the easiest and most efficient forms of recycling possible.