How to prevent acne, has been an issue for as long as we have had skin. Even though modern science has made many discoveries, some even quite amazing, they still haven't really answered that question fully.
For many years I have been fighting acne, the same way I believe you have, or you would not be reading this. Trying to come up with my own answer to that question, and even though I have been successful in preventing a lot of it, I still find myself in a situation where I have only been successful in removing somewhere around 90% of my at any given time. It is not as easy as it seems figuring out how to prevent acne.
While trying to come up with an answer, I have tried many theories of prevention, but have always some flaws within every single approach I have tested. In my humble opinion, you will not be able to completely prevent from sticking out it's ugly "face" once in a while. But with the right approach to eating, living stress free, and taking good care of your skin, you will be able to keep it in check up to some point.
Over the years I have had the best results by just keeping a healthy and clean skin, and washing my face with an wash solution. I must say though that a lot of my worst acne attacks have arrived when I least wanted them, and most feared them, and that reasoned me to deduct that is very stress related, no matter what others may think.
I have had the worst outbreaks of acne, on the days where I really wanted to look my best, and many a school photo session have been a nightmare for me. To me that says stress is a major factor in outbursts.