Let's start with the party. If you are not ready for the party and you do your shopping very late, most likely you'll pick only gifts that are generic. You can keep the stress out in the list if you just prepare it early on. Shop as early as you can and plan ahead of time.
If you face trouble and if problems come your way in this time of the year, just shrug if off and move on. It's sad to say that sometimes worst time comes during month of December. If it happens, leave the stress behind, stay strong and continue with life, move on.
Before even the holiday kicks in, start your healthy diet to prepare yourself for some unavoidable calories and cholesterol to inject into your body.
Stick to your budget. Before you put the gifts into your cart, check if you can afford them and if they still fit into your budget. Talk to the members of the family about the food to prepare for noche buena. In this case, you won't waste your money for the food that they don't like.
Exercise. Go to the gym because physical activities really help to relieve your mental stress. But if the stress really get into your nerves, easy.. go and do some yoga. There are some other ways to cope with this stress. A yoga dvd will surely make it easier for you without spending a single penny for gas.
Lastly if you are facing a really bad stress, better seek a professional help because stress can cause many physical illness and problems to your body.