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:

• Remove fitted (wall-to-wall) carpets and replace them with floor covering
• Remove throw rugs, cushions, drapes
• Avoid washing clothes in low-temperature detergents, which do not kill mites
• Frequently change pyjamas and bed sheets
• Undress and put on pyjamas outside the bedroom to prevent mites inside the bedroom from feeding on pieces of shed skin
• Ventilate the bedroom and the bed during the day, avoiding heat and humidity
• Frequently shake out bed sheets outside the house
• Frequently vacuum all sides of the mattress
• Heat the house as little as possible

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.