Moderation is the key. No one wants to feel deprived but reasonable portions will make it easier to maintain your weight. When the pie is served, just have a small piece, no need to have second and third helpings on the same day.

Look for ways to modify your recipes in calorie count. Use lower calorie sweeteners, reduced fat ice creams, light whipped toppings, lower calorie mayonnaises and salad dressings to name a few.

Exercise more often and longer to expend more calories if you know you will be taking more calories in. If you increase your exercise during this time period, you'll reduce the damage of increasing your caloric intake during the same time. The more you exercise the more you can eat without gaining weight. Increased exercise also increases the metabolism.

Eat sensibly BEFORE you go to holiday parties. You'll be much less likely to be tempted by goodies if you're already full.

Eat slowly and savor every bite. It takes 20 minutes for the stomach to signal to your brain that you are full.
If you do eat out often, order a dinner salad, pass on the bread and munchies served and split an entrée with a friend.

When heading off to parties, volunteer to be the designated driver. Alcohol provides many extra "empty" calories that you don’t need. An average alcoholic drink consists of about 250 calories. If you must drink try a wine spritzer or, for example, a rum and diet coke. Avoid sweet frozen drinks and the sugars of regular sodas where calorie counts can approach 500 per drink.

Whenever you encounter plates of food buffet-style, snack on fresh raw vegetables and lean protein, and avoid sweets and high calorie selections. When possible, move away from the food table… you will be less likely to be tempted to eat more.

Choose water or low calorie beverages whenever possible. Avoid the empty calories of alcohol, regular soda, punch, and other fruit juices. Drink water before your meal so feel full sooner.
Eat slowly, and if socially acceptable, chat often. If your food is eaten slowly and you talk longer, you'll be full sooner and eat less.

At meals, fill up on low calorie appetizers, if possible, like salads with low calorie dressing, raw vegetables and some soups.

Balance meals away from carbohydrates. Eat more turkey and leafy vegetable; less stuffing, potatoes and bread.
Eat small meals often. Avoid skipping meals before a big holiday meal; don't "save up" for a big feast. Starving yourself before a party or get-together increases the odds that all will overeat when the festivities begin.
If the holiday party is at your house, send some of the leftovers home with your guests.
So, what happens if we want to lose weight over the holidays? Simply put, weight loss is a simple law of energy intake and expenditure. Eating more than you are using means you gain weight. Burning more than you are eating means you lose weight. There are four principles to keep in mind – Nutrition (diet), Exercise (Cardio and weight resistance training), Dedication (Consistency), and Goals. It is possible to lose fat with correct diet alone (for some people) - but the best chance of success will be to apply all these principles. Even the perfect diet plan can fail if you cannot stick to it. To lose weight, one can be careful and stick to a diet on the days in between the parties and festivities. Then during the parties and festivities, don’t deprive yourself of foods you may only get once per year but choose moderate portions and increase exercise to offset the additional calories. A diet should not be looked at as a temporary experience. It should be a permanent lifestyle change to avoid the yo-yo effect of losing weight and gaining it back.