For me, the zone always came after my formal practice regimen (warm up, scales, new technique, cool riff I wanted to work out), at the end of the formal part, I'd just let rip with improvised noodling most of the time. Sometimes it was short and ok other times I would be on autopilot and I could feel my playing intensity just amp up and it seemed my fingers were just running around on their own.
I think that once your in a good and musical head space and you're dexterity and knowledge meet that meet that opened 'head', then your fingers are just rollin' with that opened up flow of music.
