1. Decide if you are getting a real or fake tree.
Both have their advantages and disadvantages. A real tree has sharp pine needles that could potentially get stuck in paws or harm kitty in other ways. Then again, an artificial tree is not a healthy chew toy either, especially if the "needles" fall off and are swallowed. If you choose a real tree, try and make sure you get a stand that does not have much exposed access to the water. Most kitties love to drink any water that is NOT in their bowls, so you will need to make sure that they cannot easily get to the water for the tree. The water for the tree has the potential to make your cat sick, so try and make sure that the water in the stand is not exposed at all. Also make sure you vacuum up any fallen pine needles regularly. If eaten these can make your cat very sick.
2. Hang breakable and irreplaceable ornaments up high.
This seems obvious, but many people don't realize just how high their cat can reach when stretched out. Try placing them as close to the top of the tree as possible to avoid kitty breakage. And be sure to place all glass ornaments out of the cat's reach - you don't want them breaking one and getting cut while you are not home.
3. Make sure the tree is secure.
This is a tough one for our home. Our apartment is in an old building, complete with very slanted floors. Even with a secure base our tree was at a slant. It took the kitties a whole 5 minutes before they had the tree tipped over the first night (thankfully it was before we decorated). Our tree is now anchored to the window sill to prevent it from tipping over because we could not secure it otherwise. You can also anchor the tree to a wall or ceiling to further secure your tree. Additionally, if possible place your tree away from furniture that your cat could jump from onto the tree.
4. Consider leaving the tree up undecorated for a day or two.
5. Don't use ornaments as toys.
This is just asking for trouble. It may be cute at first to tease your cat with an ornament or two, but it will not be so cute when they are knocking over the tree to get to that ornament. Seriously, just don't do it. And no catnip anything on the tree.
6. Consider spraying your tree with vinegar or lemon juice.
Cats hate the smell of vinegar and strong citrus smells like lemon and orange. If you don't mind the smell spritzing your tree with either can help deter your cats from bothering the tree. You can also place lemon or orange peels around the base.
7. Avoid tinsel.
While tinsel can be pretty on trees, especially when mixed with Christmas lights, it can be a serious hazard for your cat. The shininess of the tinsel is very attractive to cats and may entice them to want to play with it. If chewed on or swallowed tinsel can cause serious health problems, such as choking or damage to the intestines. If you have kitties is it best to avoid this shimmery Christmas object all together.