Do you know most of us walk around chronically dehydrated? It's super important to stay hydrated when you are suffering with allergies so that you can continually flush your system of allergens.
Clean, pure, filtered water is a great choice.
Another is hot green tea. The steam from the tea can help to open sinus passages and the antioxidants in the tea, especially the EGCG, blocks the production of naturally-produced chemicals like histamine and igE.
Remember, best to avoid dairy products while you are congested because they just create more mucus.