Most readers of this newspaper will regularly receive invitations to apply for a credit card. To tempt them, there will be an introductory offer - 0 per cent interest for six months, reduced rates on balance transfers. And Christmas is peak season for sales of mobile phones - if "sale" is the right word. There is always a selection of "free" handsets and even the fanciest model is available at a price well below its cost of production. Much the same goes for bank accounts. The easiest way for a student to earn money is to open a bank account.

These goods and services are consumed continuously but bought only occasionally. Once you have put a credit card in your wallet, programmed a mobile phone or acquired a cheque book, you use the supplier's services day in, day out. But you do not monitor the price of these services day in, day out. If you manage your credit card purchases carefully - taking maximum advantage of the free credit periods and regularly changing card - you can borrow thousands for years without paying a penny in interest. But only a sad person would invest the necessary time and energy.