Usually, around here at least, we start seeing "live" Christmas trees about the week of Thanksgiving.
We get a "live" tree ("live" in quotes because the tree has been cut down, therefore it is technically dead) every year. We vary the tree type year to year - sometimes we get a fir tree, sometimes a spruce.
Anyway, the best way to make sure it stays nice and pretty and full is to keep it WATERED!! No aspirin or sugar or anything else - just good old water. Do not let the water level get low. The tree will get dry and brittle quickly if you do (remember....it's now a dead tree).
Also, keep it away from any type of heat - heat will dry it out.