By Chris Smith,

The substance of your interview is, of course, key. Your patter, affability and subject knowledge are essential, but arguably your attire will also play a vital role in dictating the final outcome. First impressions are, by definition, instant and it takes seconds for a complete stranger to formulate a positive or negative opinion of you based on your appearance alone.

Knowing what to wear to a job interview is an age-old conundrum. Fashions come and fashions go, but style remains, and, for both formal and informal interviews, there are a few hard and fast rules. There's no room for experimentation in your interview wardrobe, so here's a guide to make sure you choose the winning outfit for the job you're applying for.

The formal interview

You should be aspiring to dress one notch above what you would normally consider suitable for work. And that of course means the job that you're interviewing for. You could hang around the car park at clocking off time to get a clear indication of what people are wearing, but as a general rule of thumb, for both men and women, it's going to be a suit.

Suits never go out of fashion. There's always some rock star or hell-raising actor sporting a two (or three) piece on the front page somewhere. A particular trend of the moment appears to be, what I like to call the shiny suit. These are made of a cloth that looks like it could coat a frying pan and, while it's perfectly acceptable for a wedding or a nightclub, it should not be attempted for a job interview – unless that interview happens to be for a boy band.

Women

You have the choice of trousers or skirt. The rule with a skirt is that the hemline should be no more than one biro length above the knee. You can't go far wrong with black. Black is the new black after all. Navy, brown and, in the summer, a lighter plain colour are also perfectly fine.

Patterns should be avoided. Add a splash of colour with a scarf, but don't get too adventurous with the shoes. Keep heels at a sensible height. Shoes can be the female equivalent of the shiny suit. Going for a plain blouse or one with a simple stripe is the safest option.

Men

Dark, sober colours are always good and cotton wins over linen, even in the summer – linen creases ridiculously easily. Shoes should be brown or black – black with a black, grey or blue suit, brown with a brown or blue suit. Avoid mixing black and brown and always go for leather, not suede.

Similarly, avoid garish patterns on ties that can distract an interviewer. Ideally the tie will complement the whole ensemble, so it should be matched with the shirt as well as the suit. It's always easiest to go with a plain, white shirt and a non-patterned, single-coloured tie. Not one that features Captain America or Homer Simpson. The same applies to your socks and yes, the interviewer will notice.

Business casual

Some companies like to test your ability to interpret fashion etiquette by setting a business casual dress code. For both men and women, casual trousers and blazers can be mixed and matched, ties dispensed with and even shoes can be less formal. But if it seems confusing, just follow these rules:

No jeans. No trainers. No T-shirts. Business casual – the clue's in the title.

In the final analysis, if you look great, you'll feel great and if you feel great, there will be a much higher chance of you storming your interview. Whatever you decide to wear, I would recommend that you start with a fairly safe, uncomplicated canvas and add a splash, but no more, of your own personality with a well-chosen accessory.

If you get the chance to try on your outfit a couple of days in advance, you will be able to get any dry cleaning done and come up with a contingency should something either not fit, or have a rip or hole in it.

And my own personal bug bear – make sure your shoes are polished.