The must-buy feeling grows stronger as you prowl through each store, trying on the trendy shoes and clothes. At the same time, you're calculating how much you will save from the discounts.

After buying a few pieces of clothing, you walk past Starbucks and see the huge promotional poster for the new Dark Mocha Frappuccino Blended Coffee - it looks really yummy, and the smell of freshly brewed coffee is just too delicious to resist. So you decide to take a break and try a cup before you head to the supermarket for food and other essentials.

Once you get home you realise that you've bought things you never intended to buy.

You might say it is impossible not to spend money - there are just so many things that you need to have, for example, a Gucci handbag, or a Valentine's Day present for your boyfriend. But there are many people in the world who survive quite happily without these things.

Why can't we?

Because there is an army of marketing specialists whose job it is to make sure you can't live without whatever they have to sell. Companies have spent millions of dollars studying you.

Retail and consumer behaviour expert Cheung Wah-leung says savvy marketers create promotional periods with holidays.

'Ten years ago, who celebrated Father's Day? It is only in the past few years that people have treated it as big as Mother's Day. Exaggerating the celebration of festivals ... makes it so big that they create an invisible pressure, making people feel there is a need to celebrate,' says Dr Cheung, associate professor at Hong Kong Baptist University.

'We call this need recognition. It even creates a sense of loss if people don't do anything special on the day. And they feel happy upon fulfilling the needs.'

Shoppers, especially those without shopping goals, are easy prey. They are more easily influenced by promotions at the entrance to stores. Advertisements function as external clues, guiding shoppers to decide what they want to buy.

Certain stores lead customers by the nose, giving them no choice but to follow a set path around the merchandise, even if they know exactly what they want.

Supermarkets are experts in retail psychology. They place milk and bread at the back of the stores, which means when customers just pop in for the necessities, they are exposed to as many products as possible before reaching their goal. They also put expensive products at eye-level, low-cost food on the bottom shelves and tempting candy and other impulse items where shoppers have to queue for cashiers.

Now they have turned to a new strategy - every day low price or EDLP, according to Dr Cheung.

'This sends a message to customers that whenever they come to the store, there are always goods on sale. Customers think they are getting a bargain, but in fact they are making an impulse purchase. They buy products that they wouldn't buy if they were not on sale.'

Also, it seems no one can go to a shop and just buy the items on sale. There are always extra items creeping into our baskets.

Dr Cheung said another marketing gimmick is the 'once in a lifetime' experience, where people are prompted to spend more on a one-off indulgence. The wedding market is especially ripe for this strategy.

'It encourages customers to spend more for something better, prettier and fancier, and thus pushes up their expectations. It pressures people, making them feel sorry if they don't do it. But actually, all these are not necessary,' Dr Cheung explains.

Psychologists and physiologists examine why shopping make people happy.

Wong Muk-yan, a PhD student in psychology at the University of Cincinnati, believes we do it because it puts us in control of our lives for a brief moment.

'You never know what will happen at your job or your family, and these uncertainties create anxiety. Shopping is an easy and direct way to relieve stress and the worst that can happen is you end up in debt,' says Wong.

Hongkongers live in a place which constantly bombards people with pressure to buy.

Dr Cheung admits customers are in a passive position, but they don't have to fall prey.

'Know your position and watch your income before spending money,' he says. 'Don't follow trends blindly, they never end.'