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I'll have to dig up references later but dairy causes your body to produce more mucus so should be avoided during allergy season or if you have a cold. Alcohol raises histamine levels and will make allergies worse. If you want antihistamines to be their most effective, don't go out on the piss.
Some studies have shown that probiotics can help some sufferers. Your gut flora have an effect on your immune system so having the right stuff in there can actually help.
Sugary foods aren't great for your stomach (or anything else, really) and cause an increase in certain types of gut bacteria that love sugar. This is a bit more tenuous but it goes with just looking after your digestive system.
Any evidence corpus for any of these measures?