My survival strategy tends to be more of the "relax, enjoy it and get back down to it in the new year" type of strategy. Because I always have a holiday as well, it's always harder because I'm eating out more, there are beachfront bars and sangria, and really rubbish veggie options.
One thing I do do though is split those little indulgences into four categories so I can at least prioritise what I do eat. There's no point filling up on crisps and then coming across your favourite christmas treat and having that too, so I try to limit myself to the really special stuff and go easy on the stuff I can have all the time.
1. Is it something that you can't, or wouldn't, get at other times of year - Hotel Chocolat christmas chocolates, Betty's mincemeat streusel cake, christmas pudding, stollen? If so, eat it (within reason!), but try to burn it off.
2. Is it something you can get year round, but which makes christmas for you - Baileys, nuts etc? Nibble a little of it, but remember you can eat it all year round, so you don't need to eat like you'll never see it again. Don't miss out on feeling christmassy because you're "dieting", but don't overdo it.
3. Is it something that you like, but it's nothing "special" and you can get it any time - quality street etc? Restraint is the word here. Why not fill up on the stuff you can't get the rest of the time, or which is christmassy, rather than on "filler" food, even if you like it.
4. Is it something you don't particularly like, but which is on the buffet? Stay away, don't nibble, don't pick. You don't like it, so why eat it?
Other than that, I stay relatively active. This is probably easier because I'm on holiday, because I've got plenty of time and nicer weather (and longer days), plus it's a lot nicer to run along the sea front than round housing estates!
I try to make sure I'm still getting at least 5 portions of fruit and veg. Even if I've overeaten earlier I'll still try to have a piece of fruit, to remind myself that there is food out there that isn't soaked in brandy
And I'm trying to drink less than I used to at this time of year. I will drink a bit, but I've finally realised that it is possible to just have one glass of wine and still have a good time