If you want to keep your Christmas tree looking fresh and healthy, you should fill the water reservoir at the base of the tree with a mixture of water and either fresh or artificial Christmas tree preservatives. This helps to prevent the tree from drying out and losing its needles or leaves. You can also add a few drops of essential oils, such as pine or fir, to give the water a pleasant fragrance. Just be sure to change the water and replenish the preservatives every few days to keep your tree looking its best.