If it will be more than 4 hours between the time you cut your fresh Christmas tree and the time you set it up in the tree stand, put it in a bucket of water in an unheated, sheltered spot like a garage or porch until you're ready to bring it inside. Cut a fresh "cookie" off the base of the trunk before placing it in the tree stand, so it can continue to take up water. No other additives are needed. Be sure to keep adding water as long as the tree is taking it up. It will absorb up to a full gallon in the first day.
Photo courtesy of mcash70