Eliminate as much as possible all the allergens you can from your environment, principally domestic animals (dogs, cats) and house-dust mites. Some practical suggestions for eliminating house-dust mites:
Avoid as much as possible allergens you cannot eliminate from your environment, notably airborne pollens.
Find out when pollen is most likely to be in the air ("pollen season")
Consult the daily pollen count on this website and when the pollen count is high take necessary precautions by staying inside or taking antihistamines.
Avoid precipitating factors, i.e. non-allergens which can bring on an attack, principally cigarette smoke, excessive exercise, sudden changes of temperature, emotional stress.