For me, getting in the "zone" when I'm writing has been difficult (especially lately) but on a good day, when I've got some free time on my hands (HA! Free time? What's that? :P) and I feel the urge to put pen to page, I like to get out of the house and whole up in a coffee shop for a few hours and write, a steamer and muffin in hand, along with my writing tools and then, I'm off. That's how I prepare to get into the zone. It usually takes me a little while to block out the sounds of the coffee shop around me, but once I do...watch out.
I can tell I'm in the zone when my drink and snack have gone untouched for more then 5 minutes, when the minutes turn into hours and I don't even bat an eye, when my brain is moving faster then my pen, trying to bombard me with a bunch of ideas at once and my pen can barely keep up, when my hand is stained with ink, when I'm lost so deeply within the story that it feels like my characters and I are the only ones in existence. And the telltale sign that I'm definitely in the zone? When, by the end of my writing session, my hands are cramped and achy and it feels like I'm emerging from a fog, a glorious, crazy, imagination-induced fog, full of images, words, and dreams for of the future, my future, my book's future and that's when I know: I wouldn't trade that zone, that total creative high, for anything in the world.