Jamie- we put the picket fence around our tree. A few ribbons and lights woven in keeps them out from under the tree. Of course, it won't keep them from making a leap and climbing up!
It’s the climbing that we are worried about. I’m not putting anything under it until the night before. The tricks I know; tie the tree up! Use three fishing line ties from the top of the tree tied off to nails above the tree in the ceiling or wall. Strong test line is needed if you have multiple cats, rambunctious dogs, or big dogs that seem to have lead pipe in the tail!
Tie on the shatter resistant ortiments. Again, fishing line, or the wire hooks you can twist on at both the bulb and tree.
Keep them distracted, give cats new toys that are very attractive, like featherd wands, catnip mice, etc. orange essence is said to be repulsive to cats, I haven’t seen evidence of this, but I’ll try it.