Treat the cause. As most asthma is caused by allergies, the best solution is to treat the allergies, as it is impossible to avoid all allergens. I suggest lda (low dose antigen therapy) or srt (sensitivity removal technique). They are both able to eliminate allergies to foods, chemicals, and inhalants.
Avoid triggers. Avoid triggers--if you know you're allergic to something--like dogs, cats, certain pollens, you are best off avoiding contact with those things. That is first and foremost. Then, if you have exercise induced asthma, you should use a beta agonist inhaler 30 minutes before exercise. Finally, inhaled corticosteroids are used to prevent asthma exacerbations. Talk to your physician about this.