First and foremost try to get your tree from a cut it yourself lot... IF you cant do that then you need to get it early ! This will quarentee your tree hasnt been sitting with out water for more than a month.
Most tree harvesters cut them down end of oct/ealry november.
Cut at LEAST 1 -2" from the base of the tree (truck) This takes off the cap the sap has made to preserve what little moister the tree has left. Make sure to put it in water shortly after you cut the bottom.
NEVER let the trees water basin dry up.. This will again cause the sap to create a cap and it will then not suck the water it needs to maintain its freshness.
When picking your tree make sure to look for brown nettles. It will have some in the center of the tree that is normal but when its alot of brown pass it up. Also grab the end of one of the branches and firmly streach it a bit and take your fingers and rub the neetles the wrong way if they easily break the tree is already dried out and will not regain any moisture. If they are flexible the tree is still fresh.
Thought id pass this on to you real xmas tree buyers!