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.
Identify the trigger. Chronic phlegm can be a sign of post-nasal drip from allergic rhinitis or sinus infection. It may also be related to gastroesophageal reflux. A physician will be able to identify the specific cause so proper treatment can be offered.