Know your triggers. Identification of allergic triggers, done by time association and confirmed with skin testing, specific ige test or a supervised medical challenge is the first step. After confirmation, do as much as possible to avoid exposure to the triggers. Antihistaminic reduce the severity of the reactions.
Allergic Reactions. Theoretically, the only way that you can decrease the risk of allergic reactions is to try and avoid the allergen which you are allergic to. If that is not sufficient and you do not want to take allergy medicines, you may want to consider desensitization/immunotherapy or "allergy shots". This is in regards to immediate hypersensitivity reactions and not delayed reactions to allergens.