I read that sit-ups give you a thick waist. Is this true? If so, what are the best exercises for getting a slim waist?

I have had many people come to me frustrated that, despite having done a variety of abdominal exercises, they have a thick waist. Obviously this is not the desired result. While a cosmetically appealing flat and firm abdominal area may be what we all covet, abdominal exercises should also create support and strength through the torso, improving posture and helping to prevent back pain. To achieve both there are two techniques you need to master. Once cracked, they deliver results fast.

The first is the rib-hip connection. This creates a nipped-in waist for women and a tapered, toned torso for men. To get to grips with this technique, place your hands around your own ribcage, fingertips below the breast line. Take a deep breath in, feeling how your ribcage expands. This is how most people start their abdominal exercises. Don't. To correct this, keep your hands around your ribs and breathe out, feeling your ribcage "lower". Now the important bit: from this position imagine you are wearing a corset (men, too!) and it needs to be tightened a little more; to do this "drop" the ribcage away towards your hips. This is the rib-hip connection and the position you need to be in before abdominal work. New mums are prone to this expanded ribcage/broader back, as during pregnancy the ribcage lifts and expands to accommodate the baby.

Secondly, master the lift-and-lengthen. Imagine you are in your classic sit-up position on the floor with rib-hip connection engaged. Start slowly to lift two inches from the breastbone (not the head, chin or nose), lengthening through the crown of the head as you lift and lower. Master these and your midriff will soon be something to be proud of.

· Joanna Hall is a fitness and exercise expert (joannahall.com). Send queries to: Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).