Do I look cool in these? Or do I look a dork?” Every man has to ask this question about his sunglasses. If he has any sense, he'll make sure that he does it in the shop - with a ruthlessly objective wife, girlfriend, or mate - rather than a week later when the lenses are scratched and he's blown £150 on eyewear he belatedly realises makes him resemble a low-grade Albanian pimp.

Not that the low-grade-Albanian-pimp look is necessarily a bad thing, if that's the persona you're trying to convey. The thing about sunglasses, perhaps more than any other accessory, is that they allow you to transform yourself, ultra-fast and with no effort, into whoever you want to be.

You want to be the Blues Brothers, Tom Cruise in Risky Business, or something out of early Brett Easton Ellis? Get some Ray-Ban Wayfarers. You want to look like one of those almond-eyed aliens who suck up sleeping Midwesterners into spaceships? Then it's probably a pair of Oakleys you need. You want to look like the Velvet Underground in 1966? Yes, I do too. So you go to Google images, as I've just done, check out that famous Gerard Malanga photograph, then try to get a similar pair with very dark lenses (Ray-Bans again, are they?) and make sure you wear them especially indoors.

It sounds easy but there's a major catch: the gap between Steve McQueen embodying cool in his Persols and Alan Partridge embodying a prat in his mirrored shades is an extremely slim one. Which way you end up depends on three key things: your degree of self-confidence, your general dress style and your face shape.

The self-confidence bit is the most important because it trumps everything. We all have friends who can put on any item of clothing, no matter how ludicrous or outré, and look brilliant. This is because, having no embarrassment or self-doubt, they are able to dress with such infectious conviction that everyone assumes they must know what they're doing.

Most rock stars have this quality. As do David Beckham and Nicky Haslam. If you don't have this revolting self-confidence thing - good on you! I don't either. People who do are mostly shallow and worthless and will surely be punished by God in the afterlife, even if in this earthly one they're destined to have far more sex and fun than we do. So it's important to choose sunglasses compatible with your general dress style.

This is what Gordon Richardson, head of menswear styling at Topman, calls: “Not stepping out of your style zone.” Thus, if you're a chinos-and-polo-shirts type, don't think a pair of dramatic sunglasses are suddenly going to turn you into Pete Doherty.

“Trying to look like a rock star is the single biggest mistake men make with sunglasses,” Richardson says. “Big, bold shades are fine if you're trying to ward off paparazzi, but on the average guy they look ridiculous. You have to work within your style zone - so, if you dress quite classically, you should go for a classic, slightly old-fashioned shape rather than plastic-framed wraparounds. This does not mean you have to be boring. Within that classic shape, there are all sorts of colour variations to explore with the frames and lenses.”

One of the best-value options this year is the Topman Classic Sunglasses Project. Five hot menswear designers were asked to develop sunglasses based on classic shapes. Japanese designer 0044 has opted for visor-like wraparounds, Oliver Spencer has crafted a neat variation on the Ray-Ban Wayfarer, Linda Farrow and Kim Jones have twiddled cleverly with the Aviator, while Bernard Willhelm has gone the Jackie-Onassis-meets-Seventies-Elton route. They are all priced at a reasonable £50.

At the more upmarket end of the scale, Prada, Tom Ford and Gucci are such covetable names that even if they got Homer Simpson to head their design teams they'd still sell like hot cakes. Another, increasingly chic make is Linda Farrow - who also make sunglasses for Raf Simons and Dries Van Noten. But this trainspotterish obsession so many young men have with acquiring the hippest brand is all wrong, says Simon Jabolin, Linda Farrow's MD. “The UK domestic market is driven by branding, when it should be driven by what fits best and looks most right.”

As for trends, Jabolin says that Aviators remain an excellent bet, but that where sunglasses are going is smaller, more intellectual and a bit geeky. “Sunglasses fashions follow clothes fashions, so you need something to go with that high-tailored, slightly gentrified look.”

And is it worth paying lots for your sunglasses? Depends, Jabolin says. There are mega-expensive brands - though he'll name no names - that charge you a fortune for what are cheap, mass-produced frames made of moulded plastic. As for UV protection, there is no significant difference between that offered by a pair of £150 Tom Fords or Pradas and those you pick up for a fiver from a stall at Camden Market. This, explains Jabolin, is because the UV protection involves dipping the glasses in a cheap formula that all manufacturers can afford. It's compulsory under EU law, so all sunglasses are safe.

But, of course, when you pay more you generally get better quality. The frames will be made from durable, handmade acetate rather than being cheaply injection moulded (usually handmade acetate ones are identifiable by the “core wire” in the frame). You also get stronger hinges and better lenses. It can't be said often enough, though, that what matters most of all is not the brand or the price, but to find a pair that suits your look and face shape.

Veronique Henderson, creative director of style consultants Colour Me Beautiful and co-author of Image Matters for Men - How to Dress for Success (Hamlyn), gives advice below.

HAIR/FACE COLOURING
Blond: Avoid black frames. Choose lighter frames - grey or transparent.
Grey/ash-brown hair: pink cheeks: Avoid tortoiseshell (because brown and grey clash). Prefer grey or silver for both frames and lenses.
Reddish/brown hair: freckles/warmer skin tones: Gold frames work well, as do tortoiseshell. And make sure that your lenses don't clash: eg, brown frames with grey lenses don't go.

MATCH THE FACE TO THE FRAME

Chiselled face
(eg, Brad Pitt, Orlando Bloom, David Beckham, Sting, Antonio Banderas)
With your prominent cheekbones and square jaw, you need to soften your look by avoiding glasses that are sharp and angular. Wear more rounded glasses: eg, Aviators

Long face
(eg, Lewis Hamilton, Prince William, Ben Fogle, Will Young)
With your long forehead and narrow face, you look like you're balding even when you're not. Your face needs widening at eye level to break up that length. Choose glasses that go beyond your face, perhaps with heavy frames, which look almost too big for you.

Round face
(eg, Mick Hucknall, Jamie Cullum, Elton John, David Cameron)
With your full cheeks, round jawline (and possibe double chin), you have a feminine, romantic face. But it needs definition. Choose frameless glasses to emphasise your smiling eyes and your eyebrows, or rectangular frames.

Square face
(eg, Russell Crowe, David Coulthard, Gordon Brown)
As with chiselled types, your block-like features need softening. Choose roundish or oval-shaped glasses.