Has asthma with cough. On steroids. What can help cough?

Hi, My mother is 50 years old and has asthma from around 7,8 yrs now. These days her asthma is not that bad but she used to get cough in almost 1 month or so. Maybe sometimes she gets a little cold (even in summers) and then she gets cough, which then gets worse and worse. One doctor previously suggested some steroids like derifylin and dexona. But then using them in 2 months or 3 months is i believe dangerous (she has already taken them 5,6 times in the past). We then consultant a new doctor and he has now suggested some medicines like: brophyle, advent 625mg, enzocort, tussin ,reswas syrup and montair lc. My main question is why this cough occurs again & again and what could be done to avoid this. The medicines are not that effective as the days are passing. What is the best way to avoid such occurances of cough and what all to eat and care.. I would really appreciate any help in this regard. Thanks, Ashish