WHERE TO BUY A TREE?

Based on my research, your best bet is to buy directly from a tree farm.  If you buy from a lot, make sure you carefully administer the "tree Test" (below.)  Another option is to buy a tree online.  This might be helpful for the elderly, but it not so great for the environment, due to the amount of gas and energy it'll take to get the tree to you.  Here's a reputable online retailer in Vermont. 

THE TREE TEST

The needles should be resilient. Take hold of a branch and pull your hand toward you, allowing the branch to slip through your fingers. Most, if not all, of the needles should stay on the Christmas tree.
Lifting and dropping the Christmas tree on a hard surface should not result in a shower of green needles. Brown needles that have shed the previous year are OK. The Christmas tree should have a fragrance and rich green color. Branches should be pliable and bend without much resistance.

Check out the "10 Myths About Buying a Fresh Tree"

ONCE YOU GET THE TREE HOME

Cut one-quarter inch off the base of trunk if the Christmas tree has been harvested over 4 hours. This fresh cut will encourage the free flow of water into the tree to preserve freshness.

Water, water, water; and don't let the water ever go below the fresh cut base. This will cause the base to seal.  Most trees require a gallon of water per day.

The experts say plain old tap water will do the trick.  You don't need to add plant food, miracle grow, lemon or run it through the brita or anything.

Display your Christmas tree in a cool place but out of a draft. Fireplaces can dry your tree prematurely.