How to choose a Christmas tree? There is an exact science.
eHow says, Look for a tree that is the right height (keep in mind that a separate stand will add about 6 inches) and width, is reasonably even all around, has a straight trunk, and has as few dead needles as possible. The needles should feel springy, not dry and brittle. If you're going to put it in a stand, make sure it has a length of trunk that is long enough on the bottom, or branches you can take off without ruining its contours.
Or, if you're the lil' dude, her exact science is more like, "Mama! This one. This is the Christmas tree for me. It is my size!"