Not using public transportation: Even if you do have a car, try to switch off with public transportation intermittently. The less cars out there, the less pollution and greenhouse gas emissions are generated. You'll be saving the environment and money on gas as well. Further, taking public transportation is even known to help you lose weight as well.
Not unplugging your electronics: Leaving your electronics plugged in when you're not using them means that the gadgets will keep sucking up the energy when not in use. The average household actually loses about $200 a year from vampire energy usage, so make sure you're unplugging your appliances when you're done. Here's a tip: try to attach your gadgets to the same power strip so all you need to do is switch it off to disconnect several appliances at one go.
Not bringing your lunch to work: If you keep eating out every day, not only are you wasting money, but you'll also be adding more waste to landfills because of the disposable packaging used to contain your to-go food. Make your own lunch and use tupperware and cutlery that you can easily wash and reuse again.
Not borrowing from and sharing with friends: Start making it a habit to share and borrow items from friends. For example, pass around a book or magazine that you guys are both interested in. There's no point in buying items (like power tools, a fancy dress, DVDs, etc.) that you will only use occasionally, because it's an unnecessary expense that will just add to the clutter in your home. Read these guidelines for borrowing from your friends without annoying them.