A coat is not only one of the major investments of the winter, but - depending on its length and which trend it picks up on - it will also impact on many of your other fashion purchases.

If you decide, for instance, on a ladylike, Jackie O/Audrey-inspired, on-the-knee coat, every other piece you buy needs to work with that length. An inch or two of dress/skirt hem showing beneath a coat is just about acceptable - Marc Jacobs made it look especially desirable in his autumn 2010/11 runway show - but any more hem (or the wrong kind of hem) hanging out and it looks like a case of dressing in the dark.

The reverse isn't quite so restrictive: a shorter skirt under a longer coat, which this season means just below the knee, or maxi length coat rather than mid-calf, also works in that hippie-chic kind of way, as gorgeously demonstrated in the 3.1 Philip Lim collection.

On top of such superficial choices as Mad Men-or-military, cape-or-camel, there is also, as a UK resident, the warmth factor. We know with a fair degree of certainty that at least some days in any given winter will be bitingly cold, so owning a winter coat is a necessity rather than a lifestyle choice, like, say, choosing between a bikini and a one-piece, or wedges and platforms…

And then there is the requirement for your coat to look totally fabulous. From November to March, we arrive places wearing a coat - restaurant, party, bar, business meeting, theatre, date, school gates - and our coat is what makes that visceral first impression, the "wow" moment, so a quiet little "please ignore me" beige number is a crime under the style police statute requiring fabulousness at all times.

Let's go back for a moment to that earlier phrase about a "major investment". The received wisdom among the fashion commentariat, of course, is to buy the best coat you can afford. And if you regularly look at the "minted" rather than the "skinted" items in those pricey-original vs cheap-copy comparisons in magazines, then it is positively your duty to buy a fabulously expensive coat - with George Osborne's cuts about to bite, someone has to.

But even "skinted" girls can actually do rather well on the high street these days. The styling is often excellent and the fabric isn't always poor - indeed, sometimes a coat at £150-£200 from the high street can be better quality than a coat from a designer diffusion-range at £500, where they have economised on cloth hoping the label will delude us into thinking it must be high quality.

You may also want to spend less on a coat on the grounds that you intend to buy two, or even three, that channel different trends and work with a variety of skirt lengths as well as with trousers.

There's certainly no shortage of trends. There's vintage/nostalgia, taking in the aforementioned, ladylike Jackie O/Audrey-inspired numbers, as well as the 1950s coats with fur collars, which, of course, reflect the whole Mad Men ethic. Do take care, however, with this look - keep it pared down, keep hair natural and the make-up soft; you want to channel Betty Draper or Joan Holloway, not Elsie Tanner. If Mad Men is your idea of fashion heaven, look for the chocolate-brown tweed with vintage-looking fur collar, £175 at Topshop; French Connection's sensational blonde wool coat with wide fur collar, £220; a Warehouse red tweed with fur collar and bracelet-sleeves, £100; and black wool with sumptuous fake fur collar from the Star by Julien Macdonald at Debenhams, £120.

For the alternative 60s vibe, Apricot's demure black coat with big collar and big buttons is distinctly reminiscent of Jackie O, despite a price tag of just £59. Also in that mood is a collarless purple swing coat with big buttons, £30 from Heatons; a camel-tone collarless coat with a distinct Grace Kelly vibe at Wallis, £110; and a fitted black coat with colour block skirt, £100 by Ben de Lisi for Principles.

Also strong in this season's pick'n'mix of trends is the cape. They look cool, hide a multitude, and if you can get past the issue of not easily being able to use your hands, put a bag on your shoulder, etc, they work as well over jeans and skinny trousers as over a skirt. Just do not wear a knee-length cape with a longer skirt, or the style police will issue an arrest warrant. Look for great capes at Reiss, which has a severe navy, zipped version at £195; Esprit with a softer, shawl collar version in two-tone grey; Warehouse, which has one in a bold grey blanket check; and Jane Norman, which has a stylish, double-breasted navy, military-style cape.

Which conveniently brings us to the season's other huge trend - military. There's always a risk of looking a little fancy dress, so rein in the allusions to militaria. Gold buttons are fine, but absolutely no braided epaulettes, please. The best military coats on parade include a navy melton at Bhs, £70; Hobbs's gold-button double-breasted, £239; low-key grey military-styled at Long Tall Sally (£95); a short navy pea coat with gold buttons at Wallis; a high-neck coat with tulip skirt and military buttons, £99, from Marks & Spencer's Autograph collection; a divine single-breasted military coat by Lanvin, £2,380; and an impeccably on-trend military coat with stand-up collar by Balmain (£3,875, both at Browns).

The maxi coat, which made an appearance at Sonia Rykiel and at Marc Jacobs, is available on the high street at Primark, which has a very passable grey maxi, available mid-October, and priced at £45.

And if you don't fancy any of the key trends, frills and embellishment make a charming counterpoint to capes, Mad Men and military. Look out for a pretty grey coat with waterfall frill at the neck and full sleeves in the Betty Jackson Black collection at Debenham's (£130) and an enchanting air-force blue coat with fluting at the neck from French Connection, at £160.