It's tough, actually. A chronic bronchitis is defined as recurring bronchitis each year for at least two consecutive years, no one knows about the exact causes. Suggestions are very general, including regular exercise, balanced diet, staying away from air pollution. There are herbs & supplements available for lung and bronchitits, can't say they will work for sure, yet you may have a try if you want. They shouldn't do any harm, if they work somehow you earn big.