Just as you freshen a cut rose by trimming off the end of the stem before putting it in water, trim an inch or so off the bottom end of the Christmas tree to open up the pours in the cambium layer immediately under the bark. Or take a paring knife and trim off just the cambium layer which is where all the water is absorbed. Then place the stem end in hot water and watch the tree just suck it up. This way you keep your tree fresh and much less of a fire hazard. But remember to keep checking the water level in the Christmas tree holder.
Big trees drink a lot.
Keep the mice at bay. But don't let them fatten up on your best trees. In other words rodent-proof your saplings. Mice and voles love to eat the tender bark around trees and will quickly girdle them, given the chance. So place plastic tree guards, available from garden centers, around the trunks.