Flushing out the nose with a large volume of salt water is a great treatment for allergy symptoms. It removes allergens and irritants from the nose and also helps remove thick, sticky secretions that may be clogging the nose or leading to drainage.
This method has been around for a long time. You may have heard of the neti pot method. It uses gravity to force the liquid through the nose. Modern technology has given us simple plastic squeeze bottles that can provide a bit more force and flow through the nose. You can find one at any pharmacy.
Be sure to not use plain tap water to irrigate the nose. It is too dilute and will irritate the lining of the nose. To make a proper saline solution, add a teaspoon of salt to 8 ounces of lukewarm tap water. If you like you can use a half-teaspoon of salt and a half-teaspoon of baking soda instead. Some people prefer one over the other, but either way is fine.
Next, tilt the head forward and slightly sideways and place the nozzle in the upper nostril. Do not plug up the other side of the nose, simply let the solution drain out of the lower nostril. Obviously, one would do this over the sink or in the shower. Flush til the secretions clear. Once or twice a day is usually sufficient.
If you have had sinus surgery, this method may not work so well. The solution can pool in the sinuses and run out at very inopportune moments. (I’m speaking from experience here.)
Also, despite warnings in the commercially available squeeze bottles, you do not need to buy a fancy expensive mix. Whatever you have in your pantry will work just fine, including iodinated salt.