Have never tried keeping cats or kittens out of the tree, a tree with dangly things on it will of course be of interest to cats. I just use non-breakable decorations and let them have at it - pretty much like this website http://www.fluffytails.ca/christmas.asp, if you want to keep it in order then you'd be best to have it in a room the cat can't go into.
We have an artificial tree now, which is less attractive to cats than a real one, and use unbreakable ornaments with no tinsel or lights on wires.Jamie "undecorates" the tree just about every night.
I have a fake tree 90% of the baubles are unbreakable but I do have a few glass ones at the top, and I have it stood on a box so they walk around the bottom of the tree, and they havent really bothered with it, mine are both 4 and its their 1st tree for 3 years