Re: Chronic post nasal drip exacerbated by water and food?
Your symptoms are very characteristic of acid reflux - specifically a type of acid reflux called LPR (laryngopharyngeal reflux) that can cause post-nasal drip, chronic throat clearing and a variety of other symptoms (including sinusitis).
Your best bet would be to get on a regimen of acid-reducing medications (PPIs like PrilosecOTC or H2 agonists like Zantac) that will help reduce the amount of acid in your stomach.
You may also want to look into the non-medical ways of improving symptoms of GERD by changing your diet, raising the head of your bed, etc.
It takes time for the symptoms to recede but over time your post nasal drip should improve.