Depends on what the allergies are.
With food allergies (often food intolerances rather than food allergies), I suggest avoidance of the offending foods until the gut wall has healed. This typically takes at least 2 months. Prime suspects are gluten, dairy, and soy. But stress of all sorts along with prescription and otc meds and alcohol (particularly red wine) can cause damage, as well.
What happens is the gut wall becomes permeable allowing food particles go into the bloodstream undigested. The body then creates antibodies to the specific food so that you now have immune reactions to that food. That is how a person becomes intolerant of more and more food as items which were once o.k. to eat are consumed and the damaged digestive system cannot assimilate them appropriately.
Once the gut wall has had time to heal, I suggest rotating foods on a 4 day rotation. Avg retention time in the human body is 56-58hrs. Thus, if you eat a food on Monday, it has time to completely pass through the body by the time you eat it again on Friday. In fact, if you like it, rotate it. Otherwise, you risk eventually developing an intolerance to that food. Pay special attention to protein based foods. In nature, the things which kill us are often protein based--fungi, parasites, bacteria, viruses--and that is what the body is trained to recognize as an invader.
In regards to seasonal allergies, I suggest checking hydration status. You should drink half your body wt in lbs in oz of water each day (i.e. 140lb person should drink 70 oz). Dehydration causes histamine levels to rise, often resulting in the creation of or the worsening of existing allergy like symptoms. Thus, consumption of sugary, caffeinated, or diuretic substances may contribute to problems with allergies. Additionally, I'd add a pinch of sea salt to the water as it's a natural anti-histamine (down regulates adrenaline, etc.).
Lastly, stress is cumulative in the body and everyone has a specific threshold over which the body can no longer tolerate. Therefore, addressing all possible lifestyle factors (i.e. thinking, breathing, drinking, eating, moving, sleeping) to make sure they are not detracting from vitality and adding stress to the system will result in a person who is more resilient and had more left in reserve when the allergic stimulus cannot be avoided. The most common stressor is a blood sugar handling issue. If hypoglycemic, symptoms will be more severe. Thus, balancing macronutrients and frequent feedings (along with a host of other factors which impact blood sugar levels) will be an important aspect of controlling the body's response to a particular allergen.
Hope that helps.