I always get a live tree. Here's what I've learned in about 25 years of buying trees off the lot:
Check for needles falling off when you shake it.
Bend upper branches to see if they feel elastic or stiff. If they're stiff and brittle, it's too old and dry.
Don't let them cut the bottom when you buy it, do it yourself when you get it home. You have a window of about one hour before the bottom starts sapping up and then it won't drink. You must get it in water within one hour. Cut an inch to two inches. If you have enough trunk, two is better.
DON'T put any kind of sugar, sprite, syrup in the water. They clog up the tree's ability to drink the water. All a tree needs is to stay moist on the inside. It doesn't need preservative or sugar, just water.
I learned this one the hard way. I had most of my lights on, and I use a lot of lights. the tree wasn't drinking. I called Captain Jack of "Christmas Tree Network" http://www.christmas-tree.com/ He is an expert on live Christmas trees. I asked him if there was anyway I could just drill holes in the side of the tree to allow the tree to drink. No. the tree would gum up worse where you drill. I had to take the tree down, resaw the end of the trunk off and get it in water. I asked him about the sugar water, sprite, preservative, etc. He said they add to the clogging up effect and to use water only. Ever since I followed this advice, my trees are fresh into January.
Don't let the water run out. The first couple of days you need to check it several times. After that check it daily.
I still use some of the large, C7 bulbs because I like the old fashioned look. They are warmer than the modern lights though. If you use any of them, it's a must that you keep the tree from drying out.
Try not to put the tree by a heat source or by a vent that blows the heat into the room.
Hope this helps.