Physiotherapy to help get rid of mucus
BMJ Group Medical Reference
Many doctors recommend that you have physiotherapy and learn exercises to help you get rid of the mucus in your lungs. But we don't know how well this works. There isn't enough evidence to say.
There are different types of exercises. You or your child might be asked to move into different positions, so that gravity helps the mucus to drain out of the lungs. The physiotherapist might tap your chest to loosen the mucus and help it start to move. Some people find it helpful to blow into a small device that helps them to loosen the mucus.
You might hear your doctor call this treatment bronchopulmonary hygiene.
If your child has bronchiectasis, a physiotherapist can show you how to help them to do the exercises at home.
A very small trial of 20 people who had regular chest phsyiotherapy found their symptoms improved. They were able to cough up more of the mucus from their lungs, and exercise more than when they were not having physiotherapy. But it did not help them breathe out more easily. 
There hasn't been enough research to show which exercises are helpful, and good studies are needed. But doctors and physiotherapists usually recommend that you use these exercises. That's because they seem to be very helpful for some people. And they don't seem to cause any harmful effects.
For references related to Bronchiectasis click here