That mad, sad moment when we decide to shop for the life we wish we were living - because, "it's such a bargain", "it might come in handy" or "just in case I'm invited."

But let's face it, in the present climate, we need to be more discerning when parting with our hard-earned cash, and thinking before we shop is always a good idea, however much money we might or might not have in our pockets.

So before you head off to the sales it might be wise to listen to the advice of someone who really knows her stuff – my latest TV fashion hero Mary Portas, star of BBC Two series Mary, Queen of Shops.

According to Mary, the best sales buys are not to be found in the cheaper high street shops but in department stores.

"The clothes are cheap enough pre-sales in a lot of shops and you will just end up buying lots of rubbish that would probably come to the price of a de-cent handbag in a designer sale. Department stores do the best sales.

They have to make space for the new collections which come in at the beginning of August, so they need to get rid of old stock.

To prepare for battle, set your alarm clock to avoid huge queues at the tills or go late in the day. She advises: "Start your shopping expedition early in the day. You will have more energy and the sales staff will be keen to help.

"You probably want to get in and out as quickly as possible, but make sure you are focused and don't go into a shopping frenzy, leaving the store with lots of things you don't need."

Rebecca Haynes, personal shopping manager for Harvey Nichols, recommends the following: "I know it sounds cliched but sales are great for buying investment pieces at reduced prices. Even though it may be tempting to buy the season's key items at a bargain, too trendy pieces will go out of style quickly and end up being an expensive purchase. I recommend my clients invest in accessories as they tend to have longevity."

"Classic tailoring and a focus on luxurious materials and detailing will be big this autumn and well-fitted trousers and cashmere are also good buys. Know your style and don't buy too extreme seasonal pieces."

So what else will be in fashion this autumn and winter? Shift dresses, lots of layering, plenty of tartan, strong accents of purple and tweeds teamed with faux fur trims. Other key pieces continuing into next season include prom dresses, 1970s wide-leg trousers, skinny belts and jewelled cuffs.

Sue Harvey of Harveys department store, Halifax, says you should always look out for the essentials – tailored trousers, well-cut white shirts, black cardigans and a leather handbag. And invest in the basics - vest tops, tee-shirts, evening tops and jeans. Stock up on these and you can't go wrong!

Buying in the wrong size because your size has been snapped up is a classic mistake she says. Will you really pay to get it tailored to fit you? Leave very high fashion items alone.