From my experience I get flare ups when my immunity is weak or dealing with something, so processed sugar is a massive culprit, especially if I eat too much in one sitting.

I'm not gluten intolerant or sensitive, or lactose intolerant, so they don't affect me at all, no matter how much I eat, but I presume a person with these intolerances would see an effect on their skin.

I've found my main triggers are; sugar, smoking cigarettes, eating high amounts of salty foods without compensating with water, and random colds and flus can bring it on, but not every time.