I've learned of a couple different ways of being in the zone. One has to do with drugs so I wont be telling people about that. But for instance, my major obsessions include "sports video games" and writing. Now it takes patience and attention, when they combine, you're almost assured it. You can't be thinking about getting in the zone, you are correct that it just happens. You pretty much have to just clear your mind and focus on what needs to be done to satisfy your obsession.
For example, when I write, I dont tell myself "this is what I want to write about, I'll sit down for 30 minutes and if nothing comes then I'll quit". In fact, I never give anything I write a title until its completed. I don't write from my mind but from my heart/soul. Yes the brain chooses the words I use but I feel it out. Being in the zone is like perfect harmony with what you're doing. When you're running and you just feel how the strides are perfectly in sync with your breathing. When nothing seems to be going wrong and all you can think about is how everything is so easy. You can't allow worldly matters to preoccupy your mind.
If you want to know the other way, feel free to PM me. I will not condone speaking in a public forum about private matters of any kind.