Answer: The best way to have a fresh tree is to cut one at the last minute, but when that isn't an option, we do the best we can.
In my experience the main thing is to keep it as cool as possible for as long as possible and keep it in water. The usual suggestion of keeping it outside is intended to help keep it cool -- and the usual suggestion of keeping it in the garage is intended to help keep it out of drying winds and direct sunlight. In addition, once you bring it indoors to set it up, keeping it away from heat sources such as radiators and heat vents is always a good idea.
The callus will prevent the tree from absorbing water, so a fresh cut is needed. The tree might soak up anywhere from two pints to a half a gallon a day, so be sure to keep the stump cut in water. If the water level is allowed to drop below it, you will need to recut it.
Finally, some folks "swear by" the various commercial products or home-made recipes for "freshener" but to be honest I have had mixed results. If it makes you feel better, go ahead and give it a try.
Good luck with your tree!
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