To make your Christmas tree last as long as possible, try spraying it with an anti-desiccant like Wilt Pruf, Vapor Gard or Cloud Cover. These horticultural anti-desiccants (also called anti-transpirants) cover the tree’s needles with a thin layer of organic biodegradable oil that reduces moisture loss due to evapotranspiration and thus prevents them from drying out too quickly, delaying their fall.
That said, the most important way of keeping your Christmas tree green will always remain, of course, making sure its reservoir contains water at all times, because if water is no longer moving up through the plant’s vascular system to hydrate the needles, your anti-desiccant treatment will be vain.
Note that you could get the same effect by spraying your tree with hair spray, but… not only is the product highly inflammable (a side effect you certainly don’t want for a Christmas tree!), but also pollutes the air with volatile organic compounds. (Don’t get me started, but the beauty product industry rarely seems to care a fig about the environment!)
Since you won’t want to cover with your furniture, floor and walls with a thin layer of oil, however biodegradable and organic it may be, apply the anti-desiccant outdoors and let the tree dry for about an hour before bringing it into your home.
Can’t find an anti-desiccant in your local big-box store? Try a garden center. It’s the kind of product specific to gardening that they specialize in. (Anti-desiccants were originally designed to prevent water loss in fresh transplants and winter desiccation in conifers.)