What I do when I get my Christmas tree is, I cutthe bottom inch or so off. This allows the tree to drink in the waterbefore the bottom cut has a chance to dry out and 'seal' itself. I alsoadd some sugar to the water; this is supposed to help keep the treefresh (though it could well be an old-wives' tale...it was simplytradition in my family and I kept following it). There are alsoproducts on the market that you can add to the water that are designedto do the same thing; I have never used any of those however, socouldn't really say if any of them help. Keep the tree awayfrom any sources of heat, have a stand that will hold a lot of water(you don't want it to dry out at any time), and if you have lights onit that emit any kind of warmth, limit the time that you have them onso they don't dry out the branches. (The newer LED lights are great touse.) I also tend to lower the thermostat in my living room by a fewdegrees during the time that I have a tree up in the house, again toprevent premature drying out.
All live trees will lose some needles, and depending on the type, Ithink some types of trees lose their needles more than others. (I'vefound that in general, pine trees lose less needles than spruce orBalsam fir.) If you take a few needles that are on your tree and bendthem with your fingers, they should be pliable...they shouldn't bebrittle and snap in half. If they do, it's a sign that your tree is toodry. Check the water level every day and you'll know if your tree isactually taking in enough. In the first few days it should drink up aLOT of water, and might slow down a bit after that. I have a stand thatholds a few quarts of water, so even if I forget to check the waterlevel for a day or so, I don't run the risk of it going dry.
If you think your tree is drying out, check to see if it is taking inenough water. If the water level isn't changing a whole lot in 24hours, you might want to consider taking the tree down and recuttingthe bottom. A bit of a pain, but it might be worth it in the long run.
That's about all I can think of...