If you run some sort of activity that uses tickets (a carousel, for instance), consider switching from tickets to reusable tokens. My kids frequent a carousel where tickets are torn off and tossed in the trash moments later all day, every day. Hugely wasteful.
Put a recycling bin on every floor of your house. You're less likely to throw away paper and junk mail if you always have a recycling bin somewhat handy.
Resist the temptation of convenient squeeze bottles for things like mayonnaise and relish. You always end up throwing away a few servings because it's so hard to get the last of it out. Opt for good old fashioned (reusable) glass jars instead. No nation is so wealthy that it should regularly throw away food for the sake of convenience.
"The best time to plant a tree is 20 years ago. The second-best time is now." -African Proverb
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.
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.
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.
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.