Sam Dunn: Having trouble at the check-out? Let Lou and Andy help

As queues go, it was pretty hostile. Shoppers hissed with irritation and children bawled their impatience. Staff tried to calm everyone down, but they too were annoyed at the hapless customer who was causing the blockage.

His crime? He had forgotten the personal identification number (pin) for his credit card. After much huffing and two failed attempts to remember, he gave up. (A third mistake, and his card would have been disabled.) Muttering something earthy, he walked off empty-handed.

Failing to remember your pin is more than a social faux pas, however. It's a serious problem for many consumers, and can hinder routine shopping and banking transactions. It can force you to leave your shopping at the store, or even lead to your card being frozen.

Remembering a sequence of four numbers may not sound too difficult, but complications have emerged since the rollout of chip and pin machines across Britain's high streets in January. The technology was introduced to reduce card fraud (see page 21), but it has led to problems for thousands of ordinary people. In particular, the elderly - many of whom have in the past received their pensions over a post office counter using a passbook - have struggled to cope with the new system. Benefits must now be paid into their bank accounts, forcing many older people to use a personal identification number for the first time in their lives.

Those who carry a handful of credit and debit cards can also run into trouble remembering pin numbers, not to mention the security numbers needed for online and telephone banking.

The solution is not to write the pin down and keep it in your wallet. If your card is lost or stolen and a criminal can use it because your number was easy to find, you'll probablybe liable for any fraudulent purchases.

For those with wallets stuffed with different cards, one oft-touted way to make life simple is to keep the same pin for each. This should work well as long as you keep your number secret; but if a thief were to steal your bag containing all your cards and a diary, it will be quite simple for him to try your date of birth as the pin. If he is right, all your accounts will be open to plunder until you cancel the cards, but you will at least be covered for the fraud.

Passionate England football fans may look back proudly on the World Cup triumph of 1966, but they should avoid using 1-9-6-6 as their pin. Banks have warned that any such memorable date could make it easier for fraudsters to use your cards.

The Association of Payment Clearing Services (Apacs) - the body behind chip and pin - doesn't officially approve of using the same pin for various cards because it allows crooks to unlock lots of accounts in one fell swoop. But it concedes that it's better than writing a number down.

For those struggling to remember a pin, Apacs recommends memory tricks. These include taking a phrase involving a favourite TV programme, such as "I love Little Britain", and using the number of letters in each word for your pin (in this example, 1-4-6-7).

Make every effort to remember your pin but, as Christmas shopping takes off in earnest, remember that keeping it secret should be your priority.