Sorry, Lindsey and Beth for being a little late with my response.
To be honest - don't hate me - but I've never had a problem with my cats and Christmas trees. I've had 6 cats in my lifetime, and the only trouble we had was with my first cat Misty. She was two months old her first Christmas, and she climbed up into our (artificial) tree and knocked it over! Hehe But after that she never bothered with it.
We always put more breakable ornaments up high (or not on the tree at all), and made sure the ornaments near the bottom were fairly kitty-safe. My current cats tried to climb the tree during their second Christmas with me (age 15 months), and they got slightly tangled in the beaded garland and got out of there as fast as possible! They've never touched the tree since.
Since I don't have much personal advice there, I looked up some advice on websites. :-) I found a pretty good site here and another one here.
The biggest thing you have to worry about is NO TINSEL around cats. I've mentioned before that once a cat gets string into its mouth, he HAS to swallow it. Tinsel will cut up the kitty's tummy and that's not good. :-( So avoid that stuff.
Using a water bottle for discipline is an option, for the cats that respond to squirts of water. You can try the whole negative reinforcement. Spray water, make loud noises, etc. whenever the kitty goes up the tree. Praise her, give treats, rub her head when she comes down from the tree. One person mentioned putting cans of coins on the bottom branches so whenever the kitty hit them, it would rattle and scare the kitty. I think this would only work for certain cats though.
Try putting aluminum foil on the ground beneath the tree and/or at the base. Apparently some cats don't like walking on it! In the second link, someone said: "Another thing you can try is that clear plastic runner matting you can use to protect carpets ... arranged upside down around the base of the tree. It has little plastic pointy things on the bottom and *most* cats will avoid stepping on it." That's a great idea.
You can get no-chew spray (called "Bitter Apple") at a petstore. We had this for our dog. Again, this would depend on the cat, but I could see how they would not like the smell! Try spraying that on the bottom of the tree, and especially the electric cords. I also read that cats hate orange scent too, so you could try hanging orange peels or using orange cleaning products.
I think that a LOT of it depends on the cat and the age of the cat. Some cats are just wild and crazy - Beth's cats, for example. :-) Paul's first cat - Chipi - was that way too. He said they just gave up on trying to keep Chipi out of the tree, and avoiding putting breakable ornaments up. If your cat is just a hyper kitten then it might be something he or she will grow out of. Hopefully, for your sake! :-)