Reduce, Reuse, Recycle...that's the mantra of the environmentalist and by using decorations I already have rather than buying new ones I'm "reducing" my environmental impact by "reusing". We bought our artificial Christmas tree nearly 20 years ago. Occasionally I think about buying a new one but honestly, by the time I have our 30+ year collection of ornaments on it, you really can't see the tree anyway.
This is what it looks like tonight and I haven't even started putting on the Hallmark ornaments we've collected over the years. I also reuse tinsel which goes on last - one strand at a time. In the photo you can see the garland draped across the bookcase. In addition to that one, I have four more which are draped across the windows and doorways of our family room. I bought these the same year we bought the tree so they are going on 20 years, too. They were nice quality garlands, not super expensive but nice. Each year I carefully pack them away in a sealed box which resides on the upper shelf in my closet. They still look nearly as good as the day I bought them.
I also decorate with items which were handed down to me, such as my grandmother's lace tablecloths (much better quality than what you can usually find these days), or were gifts. Even though I generally use the same things each year, they still look pretty and because they are only used for about a month each year, I don't really get tired of them. Besides, I like the idea of tradition.
One thing we did buy new a couple of years ago are the LED lights on our tree. We had been using the same lights for years and years but one by one they were failing. We decided to take the financial plunge and switch to LED. We still have regular lights in a few other places but we will start switching those out, too. And I did finally have to replace the bows on the tree, too. They were starting to look pretty shabby so I bought a large roll of ribbon and made the ones you see in the photo above. I expect these to last another 15-20 years. ;)
So my suggestion for having a "green Christmas" is to rethink the popular notion of buying new decorations and having different themes for Christmas each year. Consider the appeal of tradition, invest in some nice decorations and then use them every year. You can still add a little something new once in awhile but if you keep your overall look the same you can reuse most of your decorations for many, many years.