APPLESAUCE: Use an equal amount of applesauce or another fruit puree to substitute oil in brownie, cake, bread or muffin mixes. This adds flavor, moisture, and texture with lower-calories.
EGG WHITES: Replace some of the eggs in a recipe with just egg whites. 1 egg = 2 egg whites.
FAT-FREE Milk, Yogurt, Cheese, etc: Fat-free doesn't necessarily mean healthier, but cutting the fat in some dishes during the holidays can be worth it. Use the same amount in place of full-fat products.
Don't arrive ravenous! Eat a small healthy snack (apple with peanut butter, oatmeal, etc) if you are hungry before you arrive.
Bring a healthy dish with you. That will ensure that there is at least one healthy thing for you to eat.
Start with a salad or brothy soup to fill you up before the meal.
Avoid the seconds trap. Fill up your plate with modest portion sizes and avoid overflowing the plate with food or going for a second round.
Leave the table when you are done. Lingering can lead to picking at food and continued eating. Get up and go talk to friends and family.
Keep your hands full. I like to carry around a clutch or drink so that both of my hands aren't fully ready to go and grab food.
Drink warm water throughout the event and after the meal if you feel very full. This stimulates peristalsis and gets things moving.
Try to pick one dessert to indulge in instead of trying every sweet on the table.