1/ FLUID INTAKE- Your body often confuses hunger and dehydration signals. Put down the mulled wine and down a pint of water before every meal and you'll feel fuller far sooner.
2/ EAT AT THE TABLE- Doing this gives your brain the best chance to process the fact you're eating and a stronger indication of when you are actually full. It's not Christmas food that makes people pile on the pounds, it's the excess of it.
3/ VEG AND MEAT- Load up on both of these, there's plenty of this stuff around at Xmas so you have no excuses. Both the protein in the meat and the fibre in the veg will be great for filling you up for longer by keeping your blood sugar/energy levels consistent.
4/ DONT SKIP BREAKFAST- I know your logic; miss breakfast to make up for the big lunch you might be having, but it just doesn't work out in reality for most. Keep a small but solid breakfast as part of your routine over the holidays to reduce the chance of cravings and the consequent over eating.
5/ ALCOHOLIC SHORTCUTS- Most of us will have a drink or two over Xmas and thats cool, but make wise choices by substituting sugary mixer drinks (coke, lemonade etc.) for zero calorie variations. You won't tell the difference, and you'll save a ton of calories.