If you have a fresh cut Christmas tree, please keep in mind that a tree can use up to a quart of water per inch of trunk diameter per day. Check frequently to make sure that your tree stand contains enough water. Ideally, your tree stand should hold at least a gallon. If the water level in the stand goes below the cut end of the trunk, drying sap will seal the trunk, preventing it from taking up more water. Make sure your tree is away from heat sources and that you turn off lights at night or when you are away from home.
Clean water is all you need. Additives, whether purchased preservatives or kitchen concoctions, have not been shown to be effective in prolonging the freshness of Christmas trees.