For every £10 you have in your pocket, at least £5 is set to end up in the till at Tesco, Sainsbury's or another major supermarket. It's not just about convenience – the big supermarket chains' huge buying power means they can often offer the best prices. But with wages going up at a rate of 1% a year and food prices by up to 3%, it's never been more important to make sure you get the best deal. Hilary Osborne explains how