This problem is usually related to diet and weak digestion, one has to correct the diet and in some cases take a digestive stimulant. Then the excess drainage will resolve.
(weak digestion is indicated by a deep line down the middle of the tongue, according to eastern tongue diagnosis technic)
There can be environmental factors, usually those are aggravated by a sensitivity which developed due to a dietary problem.
The diet change is often reduction of sugar, dairy. eating only when hungry, reducing the volume of food you have. eating too soon before bedtime can aggravate this problem allot.
Digestive stimulants would be recommended based on your individual disposition (which I don't know not having any other info about you)
Post nasal drip is treated similarly to sinusitis under the holistic health system called Ayurveda.
hope that helps you some.