I got prescribed the most revolting tasting drink to thin my mucus when I landed myself in the hospital, it was vile, I've just discovered you can get the active ingredient as a supplement!
The prescription stuff is called Nacsys 600mg effervescent tablets Acetylcysteine and it's horrible, the supplement is called Nac (N-Acetyl-Cysteine) 600mg.
If your mucus is thick it might be worth trying the supplement, it does seem to work.