Pretty simple really
Eat less than you normally would over this time of year 🙂
Here's some practical tips you could use for it...
1. Order a small of the Costa/Starbucks Heart-attack in a cup you'll get as your
yuletide drink, instead of a medium/large
2. Give away your chocolates, advent calendar and sweets as a daily secret Santa, 
just whatever you do don't tell them the reason you're doing it is because
 "you don't want to get fat" make up some crap about being nice and not just because
 you'd rather it went on their waist than yours 🙂
3. Drink a soda & lime in between rounds on your Christmas do
4. Order a chicken shish with half pitta bread, instead of a lamb doner
 when you're outside the kebab place at 3am
5. Buy slimline tonic, light beers, drink spirits, avoid mulled wine... 
all these little calories will help
6. Learn to ice skate - impress the family and it'll also get you more active, 
use those ice rinks popping up everywhere around your work
7. Hand deliver all your Christmas cards instead of posting them, at least the local ones...
 get walking on a Saturday 🙂
8. Go Christmas shopping, not online... but actually shop! 
You're going to spend the money anyway, so get those steps up
That should be enough to get you started, and if you've got any more suggestions 
that you think would help other people, send them back in a reply and I'll include 
them on the next email 🙂