Make sure your tree stays green and your floor clean.
Offer a drink. Place the tree in water as soon as possible after you get home. (If the trunk wasn't trimmed at the tree lot, make a fresh cut with a saw first.) If you prefer not to set it up in the stand right away, place the trunk in a bucket of water and store it in a cool place such as your garage.
Protect your floors. Put down a plastic drop cloth where you plan to display the tree and place the stand on top. The stand should fit the diameter of the trunk (never whittle the trunk to get it into a stand, as that can hinder the tree's ability to drink.) For trees 6 feet tall and larger, you want a stand that holds at least a gallon of water. Make sure the tree is straight and the best side is showing.
Tweak the branches. Survey the tree to see if any areas need pruning. If the foliage around the base seems bushy or scrawny, remove a few branches with gardening shears. Also trim any small growth that juts straight out from the top or bottom of branches (as opposed to sweeping gracefully to the sides) to make more space for ornaments to dangle. Also, prune any branches that extend too far. If there are bare spots, tie a couple branches together with green floral wire to create fullness.