Ok, this is a quick and dirty one. I read about this trick over the weekend and just tried it. Frankly it's one of those things that I don't need here in Florida since I'm only 2 miles from the ocean and the breezes keep the nasties away. I wish I knew about this when I lived in New Jersey and everyone else's pollution kept floating overhead.
1) Take your finger. Any finger. Place it between your eyebrows and press for 20 seconds.
2) Press your tongue flat against the roof of your mouth at the same time.
3) Within 20 seconds you should feel your sinuses at least start to clear.
4) Repeat as needed.
I tried it this morning and... excuse me, I'll be needing a tissue in a bit.
If you want to see the write up on it in Reddit, check out this link.