This is insane hocus-pocus but it has worked for me so far:

If your snotty sinuses stop up your nostrils, squeeze a water bottle or similarly sized and shaped object (even another person's arm) under the opposite arm to the stoppage and hold it to against your torso tightly.

I laughed when a friend first told me this. I can't argue with results, but I still can't believe it. Why would it work?