It is a year to the day since the credit crunch officially began. August 9, 2007, will go down in history as the date when simmering trouble in America's mortgage market spilt over into a full-blown global crisis. That was the day that the European Central Bank was forced to inject billions of euros into money markets, and BNP Paribas, the investment bank, suspended three of its funds exposed to sub-prime mortgages.

It's been a traumatic 12 months with more surprises than a magician's hat. Think back a year: who would have believed that thousands of savers would soon be queueing outside Northern Rock or that the freezing of the credit markets would leave other banks, as well as many borrowers, on the brink of an abyss?

Received investment wisdom has been turned on its head as market watchers have been wrong-footed at every turn. So what lessons have we learnt over these troubled 12 months?

Lesson one is that “safe” shares may be anything but. Defensive sectors such as food retailers and telecoms have always been considered stalwarts that will hold up in tough times, and some have - tobacco stocks have risen 8 per cent in the past year, compared with a 15 per cent fall in the FTSE 100 index. But other sectors, which were supposed to provide protection for widows and orphans, have been worse than useless. In the past year the fixed telecoms sector, which includes the likes of BT and Cable & Wireless, has slumped nearly 40 per cent.

Why? Because contrary to belief telecoms are not immune to the economic cycle. BT's shares fell 12 per cent when it announced a poor set of results this month. The message is not to ignore the basics such as profits and sales growth.

Lesson two is that high dividends are no panacea. Normally, when stock markets are rocky, healthy dividends are a reassurance, ensuring a payout even when share prices are falling. But in the past 12 months companies paying the highest dividends have taken a severe kicking. Alliance & Leicester, the bank, may have a 10 per cent dividend yield but how much comfort is that when its share price has dropped by two thirds?

That brings me to lesson three, which is that a contrarian approach can seriously damage your wealth.

The long-term success of fund managers such as Anthony Bolton and Neil Woodford has convinced many investors that being contrarian is the way forward. In simple terms that means buying the shares that other investors shun, which are often those with high dividend yields. But the past 12 months have proved that it doesn't always work.

The contrarian choice in the past year has been banks and housebuilders, down 34 per cent and 35 per cent respectively. Of course, eventually both sectors will bounce back - banks jumped 16 per cent last week - and contrarians will argue they were right. When they do, don't forget the past 12 months and how much they suffered. Being contrarian can be profitable but blindly following an investment strategy is not.

Lesson four is one that is wheeled out every time share prices plunge but is all too easily forgotten: don't invest in anything you don't understand. Investing in banks, for example, is still a stab in the dark as it is still not clear what nasties are lurking in the shadows.

The final lesson is that value is a variable concept. Analysts have been arguing that shares are cheap almost since the credit crunch began. It is easy to support the case. The FTSE 100 is trading on a price-earnings ratio of 11 times, compared with a long-term average of 15 times. Many of the worst-affected sectors look even more of a bargain using traditional measures of value.

But they are still not cheap enough to encourage investors to pile back in. The time to buy will be when even bears cannot resist the bargains on offer, and we're not there yet.

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