How low should you go?

That dangerous time of year is upon us. In the run-up to Christmas, perfectly sensible women throw off all decorum and start experimenting with décolletage. We're used to serial offenders Liz Hurley and Nigella making the most of their assets (at a charity event last week Nigella's DD breasts, encased in a red strapless gown, preceded her through the door by several minutes). But Our Cilla? Women everywhere winced at yesterday's pictures of Black flaunting her considerable cleavage in a black dress accessorised by a giant glitter brooch.

Personally, I sympathise. It's so easy to make a sartorial faux pas. Party frocks with a daring neckline look so good on the hanger but get it wrong and everything goes into freefall. Unlike Nigella, whose breasts tumbled out of her dress like twin ice-cream scoops, Cilla doesn't understand structure. The perfect breasts are pert not pendulous. You have to think in 3D.

I wouldn't dream of suggesting a woman over 60 should hide her breasts: the embonpoints of Sophia Loren and Helen Mirren are miraculous. The French have a lovely phrase for it: "Elle a du monde au balcon" (she has everyone in the balcony).

But for the rest of us over the age of 35 there are clear rules for exposing skin. Make sure you're well supported. Avoid Bet Lynch slashed to the waist affairs (tit tape doesn't work). Never wear a crossover neckline (you get that awful crèpey crease). And never look down.

If you're going to wear breasts, you have to do it with complete insouciance, like Scarlett Johansson. Otherwise everyone gets embarrassed. I am a complete failure on the cleavage front. I spend the whole night tugging and rearranging then panic and wrap my jacket around me self-consciously. I might as well have worn a polo neck.

Which is a pity because people really do love a bosom. It's one of life's inexpensive pleasures. Women probably spend even more time admiring them than men. "Breasts really are the new guns," a girlfriend told me triumphantly. There is nothing more fascinating than a woman who owns her sex appeal.

And in a fashion climate where Michelle Obama's demurely high-necked shift dress is the height of good taste (in the sleeveless version, arms are the new breasts), it's fantastic to see someone flaunting their Jessica Rabbit curves.

But as poor Cilla found, too much cleavage can make the difference between being perceived as a sexual being and a cartoon slapper. Cleavage is just another great accessory - it needs to suit the outfit and the occasion. Sometimes less really is more.