I know what I should do; that bit isn't hard. Steer clear of nuts, cream, booze, cheese and chocolate, fill my plate with turkey and sprouts but exercise restraint when it comes to roast potatoes, bacon-wrapped chipolatas and stuffing. Resist the temptation to collapse on the sofa calling for a refill and more Turkish delight and go for a bracing walk. Limit the excess to a couple of days and get back to wholesome soups, grilled fish and the gym as quickly as possible.

I also know what I'm going to do: namely, stuff fridge, freezer, cupboard, larder and every available surface with exquisite morsels and then attempt to express love and affection for friends and family by producing a never-ending stream of multi-course meals punctuated by impromptu snacks, until everyone, including myself, begs for mercy. At the same time, I will reassure myself that a good blow-out really gingers up the digestive system and that, while we are all eating high-quality, delicious food, we are at least not stuffing our faces with takeaways and junk.

Hmm. Christmas and caution do not really go hand in hand, whereas New Year and remorse really do. This festive season, though, there is an extra dimension to excess. Earlier in the year, a Damascene moment, possibly triggered by a birthday that saw me very firmly on the path to middle age, propelled me towards both a well-known slimming club and the local leisure centre. A couple of months later, to my surprise, I was still there; and, as autumn approached, I began to feel that I had at last conquered my inner comfort-eating couch potato. It's a long process, this "making lasting changes to your lifestyle" business, but with willpower and a commitment to boring oneself to the point of entropy on the treadmill, it does work. Much as I love the decadence of Christmas, I don't want to spend January dusting off my kaftans and living on consommé to compensate.

So how to deal with the fact that, last Christmas, we lived like kings – specifically, like Henry VIII? It all started so well, with the traditional family Christmas Eve supper of shepherd's pie – not exactly the stuff of strict reducing diets, but wholesome enough. Fast forward 24 hours, and we were well into Christmas lunch: a fillet of beef, accompanied by béarnaise sauce, potatoes roasted in goose fat, peas lightly braised with tiny onions and cream and carrots swathed in butter. A household antipathy to Christmas pudding ushered in an Eton Mess. A stilton sat on the sideboard expectantly. Cuisine minceur it was not. Delicious it was.

I'm not about to give all that up for a slice of lean white meat and some steamed cabbage. It is time to harness the fact that, while there may not be much time for Pilates come 25 December, there isn't much opportunity to sit down either. I recommend abandoning the car and strapping on a pedometer. By the time you've walked to and from the 24-hour garage for extra supplies of tonic water and emergency cranberry sauce a few times, you'll have made room for at least three mini sausage rolls and a Florentine, not to mention doing your bit for the planet. Earn extra (non-) Brownie points by jumping at the first sign of a guest in need of a drink, a cushion or an inter-feast slice of cake and running between kitchen and sitting room like a mad thing – useful for maintaining both the figure and the healthy glow of martyrdom that makes Christmas such a special time. If you're a hardy sort, I also advise dressing as if it were high summer; a slender friend tells me that her secret is to make sure that she's rarely warm between October and May, thereby forcing her body to draw on its stores of fat. (Do not consult anyone with a modicum of scientific knowledge to back this nutritional wisdom up.)

And don't entirely forgo the idea of exercising control over what you eat; there are some calorie savings that can be made with the minimum of self-sacrifice. Platters of oysters and smoked salmon will do little to expand your waistline; and everyone knows that flutes of champagne – so dry! so little! – are virtually sinless. Make one rule for yourself and stick to it: in my case, eat only a nut that you have cracked yourself. The sheer frustration of retrieving shards of walnut from beneath the sofa while cupping my wrist to alleviate repetitive strain injury will soon send me in the direction of the crudités.

Other than that, continue as normal. It may be that you can avoid the kaftan by cunning deployment of foundation garments; or it may be, that replete with vols-au-vent and marzipan, you will embrace the brown-rice-and-spinach-fest that is January with something approaching gratitude.