Set up a demilitarized zone on the bottom two feet of your christmas tree, where any ornaments you actually care about never ever go. It's better if you don't have any ornaments down there at all.
Decorating with garland is bad too, unless you pin down the bottom of it so that it doesn't dangle.
If you have a real tree, your cat will try to climb it. There's no way to stop them from doing that, you're just screwed.
Also, good luck ever wrapping christmas presents again.