It happens all the time - you spend hours lugging the Christmas tree home, wrestling with lights and trying not to drop the heirloom holiday ornaments your great aunt left you - and the second you step back to admire your handiwork a certain pupster lifts a leg to add his own personal touch to your tree. You can't blame your pet, he probably thinks you got him a snazzy indoor potty for Christmas. But you can take measures to curb this behavior before the guests come over.
Dealing with a dog that pees on the Christmas tree requires the exact same type of management, redirection, and reinforcement that potty training a new puppy requires.  Here is a protocol for managing and training the dog that marks on Christmas trees:

1. Management and supervision – Your dog should never have unsupervised access to the Christmas tree.  If you are unable to supervise him in the presence of the tree, he must be crated or gated elsewhere in the house.  Do not hesitate to go back to the potty training basics you learned of crating, gating, and tethering when you are not able to train or supervise.
2. Know the signs – Many dogs offer “pre-indicator behaviors” – behaviors that are offered consistently before the event as a kind of “tell” that the dog needs to eliminate.  This can be sniffing around, scratching at the ground, turning in circles, or even approaching the tree.  If you notice any indicator behaviors from your dog, get him out for a potty break right away!
3. Reinforce success – While you’re working on this, build a strong reinforcement history (with toys, food, and/or life rewards) for appropriate elimination outside.
4. Click and treat the absence of the behavior.  If your dog approaches the tree, sniffs it, and walks away, click and treat him!  This is the behavior we want!
5. Consider keeping your dog on a “traffic lead.”  A traffic lead is a very short leash, basically just a handle with a clasp.  If your dog will not recall reliably under every conceivable circumstance, a traffic lead may not be a bad idea.  It will ensure you are able to get your dog out quickly if you DO notice him going toward the tree as if to mark.
6. Find out your dog’s schedule.  What times of the day does your dog normally eliminate?  Provide plenty of opportunities for outside potty breaks during this time.
7. Remember what events might trigger a dog’s need to relieve himself – In general, dogs tend to eliminate after: naps, playtime, eating and/or drinking.
8. Consider use of a belly band - Belly bands are snug fitting fabric bands which are wrapped around a dog’s lower torso.  They are fitted with absorbent pads so that if your dog does mark, the liquid will be absorbed by the pad and not the beautiful, pristine white  beard of your favorite pirate Santa ornament.  You can make belly bands or purchase them online.  A google search for “dog belly band” should bring up all the information you need.  Belly bands are certainly management tools more than anything, they will not train your dog to avoid peeing on the tree but they will ensure that if management slips up, your tree remains pristine.

http://blogs.dogster.com/dog-training/what-to-do-if-your-dog-is-a-christmas-tree-pee-r/2010/12/