Water, water, water. Never let a cut tree dry out. Check the tree stand's water reservoir frequently, particularly when you first bring the tree into your home. If the water does fall below the bottom of the tree, even for a few minutes, recut the trunk about an inch up.
Location, location, location. Place the tree well away from heat sources like radiators, fireplaces, wood stoves, and hot air vents.
Recheck for dryness. On a daily basis, check for excessive needle drop or dry, brittle needles. (Needles should bend easily without breaking.) If your tree fails these tests, it is dry and has become a fire hazard.