You know how you can't take a water bottle through airport security? Empty it out before you go through, and just refill it on the other side from the water fountain. Much better than throwing it away and buying a new bottle a few minutes later.
If you want to green your home by adding/changing insulation look for companies that sell reclaimed insulation. You get to save energy twice! Check the link below; they sell all over the country.
Open your curtains in the morning to let solar energy (aka sunlight) in to warm your house, and close them at night to help keep it in. Sometimes it's tempting to close curtains during the day to darken a room, but remember that you're wasting free solar energy.
Save leftover hardware (screws, nuts, washers, etc.) from pieces of self-assembled furniture. Just toss them into a bin in the garage or basement. When you need a screw, check your extras before going out and buying an entire box of new hardware.
Save your plastic easter eggs for next year. One bag of plastic easter eggs will last you many years if you gather them up and save them. Just add new prizes (we usually use small pieces of chocolate and money.) If your kids have outgrown them, pass them along on freecycle. There's no need to repurchase them every year.
There are a number of sites that will help you remove your name from direct mail (i.e. junk mail) mailing lists. Register with them and save energy and resources being wasted on delivering to your mail box a bunch of crap you have no interest in.
Make sure to save your cup sleeve from your favorite coffee shop and reuse it. They're usually made from recycled material, but reuse is always far more efficient than recycling. One sleeve should easily last months. There are also plenty of patterns out there that you can use to make your own reusable sleeve.
Follow in the footsteps of blackle.com and make your website's colors darker. This can help save energy. For example: If you've got a centered layout (like TipBin), you could make the sides of the page darker. Every little bit helps!
Use blackle.com to search Google and "save a fair bit of energy" as they put it. The darker screen apparently helps save energy. Who knew?!
You know those 50 napkins you usually get with fast food meals? Don't throw them away when you're done. Put them with the rest of your paper napkins and use them for causal meals (probably not when you have guests over). Save some trees, and save yourself some money.