Please keep in mind that a LIVE christmast tree is indeed still a living tree. Providing minerals, nutrients, and water are all required to keeping your 'plant' healthy during the month that christmas exists in.
Simple advice:
Make sure you water your tree everyday, include some minor potting soil in the bottom of the christmas tree watering stand. Ensure that the water isn't cold, or hot, that it is at room temprature.
Include some nutrients from plant stores that keep cut stems healthy. Miracle Grow, also helps, but use it sparingly.
Water it absoutly every day.
Make sure you place your tree within a location that will recieve direct sunlight, unfiltered, which is preferential.
Other than that, make sure that the room isn't too hot, but the average room temprature, water it, and keep it healty as best you can.
The unfortunate part of this however, is that when the tree starts to 'get dry', the amount of water and nutrition within the shaft of the tree is minimal and the tree is already dying.
The best bet is to ensure that enough water, and soil is available for it to uptake. please remember also, that if you cut a christmas tree down, you need to ensure that you cut it cleanly accross. If the stem is slanted, it won't uptake water at a constant pace, but instead at a variable rate which makes the tree more suceptiable to disease and lack of nutrition.
If the tree is already dry, the only thing you can do is try to mist the trunk from base to tip with water, and make sure enough potting soil and water is available at the base. As the tree has no roots to go off of, you have to ensure proper wattering.
Aside from that, there isn't much else you can do. Remember, that by having a LIVE christmas tree, you are working on borrowed time. Once the tree is cut down for decoration the tree starts to die. There is no bringing it back to life, only maintaing it.