Take foods rich in vitamin c such as citrus fruits and guava. Ensure that the fruits are not very sour or cold or else you can take vitamin c supplements. Avoid exposure to dust, smoke and extremes of temperature. Do regular moderate exercise, stop smoking and taking alcohol if you do. Deep breathing exercises and yoga help. Take regular steam inhalations and spit out all your phlegm. Any change in amount or character of your phlegm, or increase in breathlessness, or fever or blood in phlegm should be reported to a doctor.