What are some forms of prevention for asthma?

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.