Remember to... Budget!
Budget just what you want to spend this Christmas and put the rest away in a financial institution so it will be harder for you to be tempted.
While it is easier and more tempting to shop using a card, have a ballpark figure in mind of just how much you want to spend and ensure that you do not go above this. Or better yet, travel with cash instead and leave the card at home.
Make of list of the things you need, and avoid, as much as possible, the things you want. Then stick to that list.