Hi wanna_juice

Guaifenesin is an excellent expectorant, it is extracted from the Guaii plant, and is very effective for thinning and loosening mucus.
Marshmallow will help loosen mucus also, but not quite as effective as guaifenesin.

I think vitacost.com sells it at a pretty reasonable price.

If the mucus is caused by a virus. Brew some pine needle and marshmallow tea.

Hopefully you've taken care of that congestion... some great herbs were recommended but if you are interested in doing a short (say, 3-5 day) juice fast, this will definitely help clear your lungs. A few years back I had a nasty virus that settled in my lungs. I ended up eating fruits and vegetables for 30 days as a pre-cleanse diet, then doing a 60 day juice fast! My lungs finally cleared about halfway through the fast.

Some great juices for the lungs include lemon, horseradish and garlic. You can take garlic and horseradish juices in small quantities mixed with carrot and red beet juice. You can dilute lemon juice in purified water and drink first thing in the morning. You can also take lemon juice plain, one teaspoon 2-3 x per day.

Fresh juices of carrots, parsnips, watercress and raw potatoes are beneficial to the lungs.

Oranges, lemons, black currants and rose hip tea are also good.

Be well!

Best,
Somer