One of the easiest mantras of environmentalism, and one that makes sense even to the average person who doesn't consider him or herself an environmentalist, is "Reduce, Reuse, Recycle." They are listed in the order of importance, and in the order in which they should happen for each specific object. Everyone admits at the least that it takes money and energy to create new products for people to buy, so "Reduce" is a no-brainer, as is "Reuse," but it doesn't hurt for me to remind you abut them, as well as "Recycle."
Consume fewer goods. New goods use the natural resources that go into the goods, energy is required to process the raw materials, and (depending upon the product) additional pollutants are often released in the process of creating the product. One example is Helium - all the Helium used commercially is currently mined from pockets within bedrock. Believe it or not that's the easiest place to get Helium from. Although more is being created, it's not happening as quickly as we use it up. When we do start running low, we're going to have to either reduce our usage or expend more effort (time, money, electricity, research) to get Helium from other places. If we can reduce our usage sooner rather than later it'll take longer to get to the crucial stage, and we'll be better able to handle it when we do.
As a consumer, when you buy a new item you're also spending money that you may not need to spend. People who grew up during the Depression, and people today who are less fortunate (including college students!) are already used to this. The petition linked here is urging congress to create a "Do Not Junk Mail" list much like they did a "Do Not Call" list, which would reduce the amount of waste paper and make your life a little easier.
Once you are done with an item, donate it to someone who is less able to buy new items. The reuse of items helps with reducing the original amount of raw materials needed (see above arguments for reducing). It also saves on landfill space, which is limited. Sure, we could create new landfill, but the Earth's only got a limited amount of square miles, and it's less when you subtract 2/3 of it for oceans, and other things like forests that you don't want to destroy, and farms, and places people live.
Sell your old things in a tag sale. Give them as hand-me-downs to friends or family. Or donate them to Goodwill or another charity such as your church or a homeless shelter. When you get a new cellphone, donate the old one to a local battered women's shelter, or even give it to someone who'll resell it online. There's some people who can't afford to buy new items, so by donating things to them you're giving them a chance they wouldn't have otherwise. This is another good way to give to charity if you don't have money to send to the Red Cross.
If you're willing to buy reused (thrift, second-hand) items yourself, do so. Second-hand items are often less than half the price of new items, and can be in very good condition (especially manufacturer's returns and refurbished electronics, both of which often have longer warranties than new products). Save the containers that you get foods like potato salad in, and use them to carry your lunch to work.
Ditto on who already does this - anyone else own a curb-side couch? In my home I have three main collection areas for things I am done with: trash, recycling (up next on the list), and donations. I produce around one trashbag worth of donations a month, saving me money on my trash collection, and giving me a feeling of good karma.
If an item is no longer reusable, or if it never was, recycle it. It's easier to clean up old materials for recycling than it is to get new raw materials from plants, animals, or the ground, doesn't use up the natural resources, and doesn't use landfill space.
Most cities recycle paper, cardboard, glass, metal, and plastic. Some also recycle styrofoam, batteries, and electronics. If your community does not automatically recycle these, there may be a local college or university that does, and if they have public recycling bins you can add yours. Another option is to find a recycling company and personally contract with them. If you pay for trash removal, it's usually cheaper to recycle goods than to throw them out. My trash removal costs me around $15 a year and my recycling is free. If I simply threw everything out rather than recycling, I'd probably pay $40 a year.
Not part of the official mantra, but worth mentioning is composting. Composting is usually done primarily with plant matter, such as fruits rinds, vegetable peels, fall leaves, grass clippings, and newspapers, but eggshells are also good candidates. Meat, animal fat, and cheese tend to stink too much to be worth the effort of composting. Bananas and their skins can also be a pain due to fruit flies. As opposed to throwing out this garbage, you are saving on landfill space, the energy required to transport it to a landfill, and to maintain the landfill.
If you live in a house with a garden, you can create a compost pile in your yard. Compost piles can take the form of just dumping everything in a pile, or you can buy a compost bin to make it work more efficiently. Worms and other natural critters then eat your garbage and turn it into a rich soil that you can use as a fertilizer for your plants.
If you live in an apartment or have little area around your house, consider indoor vermicomposting. You take a ~100 Qt (or L) plastic storage bin (for one person, probably need bigger for larger families) and fill it with shredded newspaper. In the center you bury a starter bunch of soil and red worms. Then you throw in all your plant and newspaper waste and water it once a month, and the worms process it as fast as you can throw it in. You can occasionally take out scoops of the dark rich soil to add to houseplants, but you might wanna pick out the worms first. I've done indoor worm composting in my apartment for three years, and I've had two successive bins only because I gave the first to a friend because she wanted to use it for teaching. The only time I've had problems with it was earlier this fall when I stupidly threw in a banana peel and ended up with a month-long fruit fly infestation. I cured it by putting the bin outside on the porch for a couple more months - I'm not sure if all the new fruit flies being hatched just flew off elsewhere when the peels finished decomposing, or if an early frost killed them (and half my worms) off, but I've brought it back inside and the bin is doing well now. It makes a good conversation piece too. ;)