The best way to keep a cat out of your Christmas tree is to use deterrents. Cats are curious creatures and they will naturally want to explore the tree. However, there are several things you can do to discourage them from climbing the tree. Try placing double-sided tape around the base of the tree or spraying a cat repellent near the tree. You can also try placing a few boxes or other objects around the tree to create a barrier. Finally, be sure to keep any cat toys or treats away from the tree so they don't have an incentive to climb up.