Salt water cleanout
Chronic phlegm is often a result of postnasal drip.
My first suggestion be to try to wash out the postnasal drip. A salt water wash out of your nasal passageways on a daily basis can be useful. There are many nasal rinse kits available over-the-counter for this. If this is not adequate, then prescription nose sprays can be helpful. But there may be other issues, so see you doctor.