There is a special breathing technique we use in Germany which was "made" by a doc in Belgium. Its called "autogene Drainage (AD)".
You breathe in deep through your nose, hold your breath for a moment and slowly let the air out with nearly closed lips and do it again and again. You will experience that the air is going "under" the mucus, loosens it and is transporting it "up" with the breath coming out. So after a while you will notice that the mucus is nearly up your throat so that you can get it out with one little cough.
While breathing in and out you should try not to cough too early, breathe calmly until the mucus is really up your airways.
Its a bit similar to using the flutter.
Uli,45, Germany