There's a lot of things that could be causing post-nasal drip, so it's hard to give decent advice without examination by doctor. If it's an allergy, the most effective meds, according to research, is pseudofed and singulair. Both work differently, so you could try each, seperately. Your phlegm would be clear or light green probably
It could be a sinus infection if your phlegm is yellow or darker green. But if you got a sinus infection without a cold first, then you should visit an ENT to make sure there's not some kind of physical blockage in your sinuses.
Apparently I am the only one here who has ever had post-nasal drip. Amazing!