Over the past few years, allergies, unfortunately, have become a part of my everyday life.
I'm sure everyone's remedy is a little different; but I've found the best decongestant solution for me is a hot shower. Why? The steam works wonders in breaking down the congestion.
While hot showers are not always readily available when backpacking, I've taken this remedy into the backcountry by utilizing Ramen soup as an alternative. Boil it up, breathe in the steam, and have some hot, sodium-rich liquid for dessert. I suppose other food items and OTC solutions can work as well, but I've found soups, in general, to be the best solution.
During activity, allergies aren't as much of a concern for me (it's mostly around camp), so I can't provide much insight on that.