Plastic loyalty cards for customers from outlets up and down the High Street are all the rage. Chances are you've got a wallet or purse full.

Sainsbury, Tesco, the Co-op, and Safeway, not to mention Boots and W H Smith, all have loyalty cards for customers, leading to money off goods and services for customers who present the card whenever they shop at these stores.

In addition, a whole range of credit cards offer points to customers too, to be swopped for free gifts and flights, discounts on holidays, shopping, cars, and even new mortgages.

It's easy to scoff at the value of these cards - especially when you start to tot up how much you need to spend to get a worthwhile discount on your shopping. The most recent loyalty league table shows that #3000 of spending means a return of just #5 with TSB's Trustcard, for example.

But points do start to mount up eventually - especially if you, like many shoppers, prefer to put as much shopping as you can on your credit card and pay off the bill in full with just one cheque at the end of the month.

There's no point spending for the sake of it and incurring interest charges just to amass points, but if you can pay for as much of your regular shopping and bills with a loyalty card and earn discount points in the process then the result can be rewarding.

You can often double up on points if you play your cards right - using your NatWest credit card, or Sainsbury's own Visa card, together with your Sainsbury Reward card, for instance, doubles the number of Air Miles you can receive and boosts your chances of qualifying for free flights with British Airways.

It's easy to forget just how valuable the points on your cards can become, as Herald reader Chris Larkin discovered.

Chris had his wallet stolen on the train while he was returning to Scotland from London. And when he got home he had a marathon session on the phone cancelling the various bits of plastic that had gone missing.

''I had emergency numbers to ring for all my bank cards, but didn't know quite what to do about my supermarket loyalty cards,'' says Chris.

''I knew I had clocked up a certain amount of points on my Sainsbury's Reward card, for example, but had no idea how many, and didn't know if I needed to cancel the card or not.''

Chris rang his local store. They advised him what to do and also confirmed that the points on his card were worth around #40 in terms of money-off vouchers. ''This was more than I'd imagined,'' says Chris. ''If whoever pinched my wallet had turned up at Sainsbury's with my card they would have been able to stock up on quite a lot of shopping at my expense.''

The message from Sainsbury's is clear: ''Customers are advised to treat their Reward card in the same way as they would their credit card.

''As soon as they realise it has been lost or stolen they should report this either to their home store or ring the customer helpline on 0500 101010. The card will then be cancelled, a new one issued, and the accumulated points transferred.''

It's important to act straight away because it can be much easier for a thief to use your loyalty card than it would be to use your credit card.

Often all you need to do to claim your money-off vouchers or discount is present the card at the checkout or customer service desk - you don't need to show any other identification - and that's something a thief could do just as easily as you.

So check with the store or organisation that issued your card for the right number to ring in an emergency.

One way to cut down on the hassle of phoning round to report each of your cards stolen is to list all of them with just one card issuer. Most offer a service where they will make a note of all your plastic cards. Then, if these go missing, you need make only one call and then the card issuer does the rest of the ringing round for you.

A service like this typically costs around #8 a year - ask your own bank, building society, or store card (Marks & Spencer has a scheme called Cardsafe) for details. And make sure that if you do receive extra cards you add them to the list as soon as possible.