Selection and Care of your Tree
How to Care for your Tree:

If you purchase a pre-cut tree, be sure to ask for a Fresh Cut or do it yourself when you get home.  If you do not, the trunk of the tree will be unable to draw in water and it will die much quicker. 

Water, water, water 
Whether you cut your own tree, or buy a pre-cut, the most important thing is to water your tree.  If you are unable to set up your tree immediately, just set it in a bucket of water.  Once you set up your tree indoors, be sure to water it often.  The first couple days, it will need a lot of water, but will eventually taper off. 

Temperature
Your tree will last longer if you keep the temperature in your house moderate - between 65-70 degrees is best.  It is also best to avoid setting up your tree near a fireplace, heat register, or vent as this will dry out the tree quicker.  If you can not avoid this, help minimize the damage by watering often.