The MythBusters tried several household methods in an attempt to prevent a freshly-cut Christmas tree from shedding its needles over six weeks. You can keep needles from falling off the Christmas tree by adding...
1) fertilizer
Busted
Fertilized Christmas trees lost the most amount of leaves and its leaves became very discolored.

2)bleach
Plausible
fewer needles lost, but the tree looked sickly.

3) lemon lime soda
in the middle

4) pain reliever.
in the middle

Hmmm. I heard 7-Up and aspirin were the for sure winners.  Maybe not. 'Guess I'll just water it and hope for the best!