I suffer from chronic allergies that cause heavy nasal and sinus congestion and post-nasal drainage. My doctor recommended the following simple remedy that has been very effective.
1. Prepare a cup of chamomile (Manzanilla) tea, normal drinking strength.
2. Pour a bit into your cupped palm and sniff it strongly up both nostrils.
3. Tilt your head far back and pinch your nose shut for 30 seconds or so, then blow your nose.
This produces a wonderfully clean and open feeling in the nasal passages and keeps them clear for several hours, better than commercial antihistamines I've tried. It seems even to reduce sinus pressure in the forehead. You have to experiment to find the best frequency of dosage -- if you use it too often you may find that your nose becomes too dry for comfort.
Free breathing and absence of drainage greatly improves your sleeping and general comfort.
Chamomile is a very good antihistimine. I use the essential oil on each side of my nose whenever I begin to feel sinus pressure, congestion, burning, etc. Works immediately. Added bonus, very healthy for skin and complexion.