A important question, too, given that so many people have curvatures of the spine. An American yoga teacher named Elise Browning has an amazing story regarding her own journey with a severe scoliosis, assessed at a 49 degree deviation in the thoracic and a compensatory lumbar curve when she was only a teenager.
Elise didn’t take the surgical option and instead found help in therapeutic yoga. I recommend her books and DVDs.
A quick answer for Sinead, in the meantime…. Backbends that strengthen the back can be useful when introduced slowly and methodically, such as Bhujangasana (pictured) or Salabasana. Also, a passive backbend with a bolster or 1-2 blankets rolled up and placed under the shoulder blades, head supported, and arms stretched over head.
As for inversions, poses that help traction the spine like Adho Mukha Svanasana done in ropes or hanging from a belt looped around a door knob. Or, positioned in a “yoga swing”, supported sarvangasana (with chair, blanket, bolster), and, the old standby, Viparita Karani, with the sacrum supported on a bolster.
When, as a teacher, you observe your student, you aim to decrease their lateral curve and decrease posterior rotation. Tractioning and anti-gravity poses work well to bring the student back into alignment, but strengthening legs, abs, and spinus erectus muscles are absolutely necessary too.