I've never had a tree knocked over but I always tied them to the wall with some very heavy fishing line. I also never put precious or delicate ornaments on the lower portion of the tree.
Excatly, NOTHING you want to keep goes on the lower half! Those tails send em flying, and they love to grab the decorations and candycanes off
(Quinn has a habbit of having to carry something in his mouth every time he goes out, can just see the tree ornaments piling up outside, lol)