Next time you realise you're sucking in your gut, don't feel ashamed. Physiotherapist Sammy Margo says it's a good habit that will protect your lower back. "Your tummy muscles are the support structure for your core," Margo says. "So during the day, they should always be about 30% contracted." Contract your stomach muscles and, she promises, you will look slimmer and taller, with a flatter stomach – and be less likely to suffer from lower back pain.

Here's how: "In the morning, stand in front of the mirror, suck your tummy muscles in as far as you can, then release by 50%. Tie a piece of string in a reef knot snugly round the narrowest point, then put your clothes on."

When you relax your tummy too much during the day, it presses uncomfortably against the string, until you learn to suck it in. "It is far better than doing sit-ups," says Margo, "because this will ask your tummy to work as it should in your normal life, as opposed to just in the gym for an hour."

Start with 50% contraction, then aim for 30%, Margo says. "You can progress it after a few days. When it becomes part of your natural posture, you can take the string off."

For more, see sammymargo.com