SPRAY REPELLANTS are effective but I'm always leary of adding artificial odors of any kind in the air, particularly during Winter when windows are closed or if a fireplace is nearby as they could be flammable.
PORTABLE ELECTRIC FENCES around your indoor tree are the newest thing on the market. Personally I have not had a need to use them.
TINSEL is a no-no. Glittery and light, they capture the slightest air motion that cats find irrestistible. Not only can it be an invitation to climb your tree or play with a nearby glass ornament, but they can ingest it which is hazardous.
MUSICAL AND MOVABLE ORNAMENTS are similar to tinsel in their enticement so avoid them if possible, or place them highest on the tree outside the view or reach of a pet.
GLASS ORNAMENTS and other breakables are risky with any pet especially a new puppy or kitten in the house, so perhaps that first year try using homemade, non-breakable ornaments. Even with these, be sure to use supervision when your pet begins his/her first exploration. Always be ready with an emphatic, "No," if they get too close.
JUST SAY NO in an empahtic voice. This is a simple and often effective training method for discouraging a docile pet from the tree.
ARTIFICIAL GARLANDS and greenery have been mistaken for the real thing and will be ingested, causing vomitting or worse, so heed caution when selecting a site for display.
LIGHTS, particularly flickering, can be a temptation so again, heed caution during the process of erecting and dismantling the tree and while it is on display. Watch your pets carefully and keep them away.
CLOSE THE DOOR of the room(s) where the tree(s) reside when you're not home so your pets are not tempted while unsupervised.
If you have any other ideas to share just email me and I'll publish them here. Remember, a SAFE HOME is a HAPPY HOME.