Enter the rag trade and see the designs you've ordered flying out of the stores.

It's catwalk season and, from New York to London, bone-thin models are strutting their stuff in creations that range from the exquisite to the extreme. The humble high street seems a million miles from this rarefied world.

But the gap between catwalk and high street has in fact never been narrower. The latest trends are now modified, manufactured and shipped en masse to the masses ever more quickly.

This process - "fast fashion" - can be highly lucrative for retailers who get it right. The budget chain Primark has leapfrogged over high street stalwarts such as Debenhams and New Look to become the fourth-largest women's retailer in the UK, largely due to its ability to shift high volumes of on-trend items.

This summer, for example, Primark shifted 90,000 polka dot dresses at a tenner each. Last year, it sold 250,000 military-style jackets at £12. Even Marks & Spencer, the UK's largest clothing retailer, which has had its fair share of problems but is now enjoying something of a renaissance, has reduced the time from design studio to store to just six weeks on some items.

In the fast fashion revolution, the key figures are the fashion buyer and fashion merchandiser. A fashion buyer for a high street chain such as Topshop or Next will work with suppliers and in-house design teams to identify and develop the trends for the next season. Buyers for department stores, such as House of Fraser, will work with brands to buy in selected items from a collection, or may work with suppliers to modify a trend that will best suit their target market.

In both cases, the buyer will have to marry a good eye for fashion with a keen business sense. Liz Dean, the manager of buying and merchandising at the specialist recruitment consultancy Fashion & Retail Personnel, says: "People tend to think buyers are really design-led. While there is an element of creativity, the most important thing is to think commercially because if the items do not sell then you will get the sack."

Fashion merchandising is even more business-led. "It's about having the right stock in the right place at the right time at the right price," Dean says.

Would-be buyers need a specialist fashion degree, but many merchandising recruits come from a non-fashion background. Merchandisers typically have a business or numerical degree, and will deal with pricing and statistical models so they can make decisions about how to maximise the profit on each item.

It can be difficult to get your first job - M&S, for example, says it is inundated with applications for these posts - so it helps to have a relevant degree and some retail work experience on your CV. Yet, while our universities churn out hundreds of fashion design graduates every year - many more than there are design openings - there are surprisingly few fashion business programmes.

De Montfort University, the University of Westminster, Manchester Metropolitan and the University of the Arts London are among the institutions offering specialist courses to equip graduates with the requisite skills to make it in this hard-nosed world.

Despite the attractions of attending catwalk shows and managing multimillion-pound budgets, this is a tough business. This is particularly true at more junior levels, where the industry is mainly staffed by women in their twenties or early thirties, with the hunger and the energy to cope with long days and extensive travel to manufacturing bases far away in Asia-Pacific.

"Fashion is perceived to be a very glamorous industry, but you might not think that when you're still at your desk at 10pm and you've got to catch an early morning flight to India to visit half a dozen factories," says Julie King, head of fashion at De Montfort University.

Despite this, King says, buying and merchandising are great graduate jobs, offering plenty of responsibility from quite a young age and the buzz of seeing the designs you have ordered flying out of stores.

What you need to get a job

Check out degree programmes at De Montfort, Westminster and Manchester Metropolitan universities.

Retail work experience is vital: look for degree courses that offer exposure to the fashion industry, either through a four-year sandwich programme or industry-based project work.

You'll need a wide range of business skills. M&S's recruiters look for initiative, negotiating skills and an ability to work closely with designers, suppliers and store managers.

Expect to start on £16,000-£17,000, rising to more than £35,000 in four years. A head of buying might command £75,000-£100,000.