In my house, the holidays are like all of the other days of the year - busy and hectic. But they also tend to come with a little bit more to do than usual and higher calories treats, which can make staying healthy that much more of a challenge. The best way to face this challenge is to keep things the same as much as you can.
For me, this means that working out (running or cycling) is still a planned activity. During holidays, I tend to get my fitness on early in the day so that it is done and I don't have to worry about something else taking over.
Meals are planned, as are snacks. Let's face it: if a plate of cookies is left out to eat whenever I feel like one, they are going to disappear. I try to decide what snacks I want during the day and when I'm going to enjoy them.
When something new is put in front of me to eat, I'll usually try it (assuming it is lactose-free, which is a great excuse to stay away from a lot of fatty foods during the holidays) without worrying or feeling guilty about it. If I do, though, I'll cut back on something else or work out a bit more the next day.
And, finally, I try to eat as I normally do. I don't eat a huge meal "just because it's Thanksgiving or Christmas". Instead, my proportions usually end up being the same.
My favorite way of staying healthy: plan a race for the morning after a large family meal. It's so much easier to stay away from the second helping when you know you have to run 10 miles the next day.