I have a list of a few fruit juices here that helps against mucus formation. One of the main fruits helpful here would be the pineapple. The pineapple juice is really helpful and clears the lungs to a great extent and thus allowing you to breath easy. Other than that mostly all fruits help in this situation as they have plenty of Vitamin C which is very essential. Apart from just fruit juices, if you are comfortable I can recommend some tomato juice too. Then there are vegetable soups made out of onions, cauliflower, celery, garlic etc. These fruits and vegetables would definitely help.