For me, I've thought about it like that quote from Layer Cake: "Meditation is concentrating the front of the mind with a mundane task...so the rest of the mind can find peace."
I got into the Zone with Perfect Dark, dual scoped falcons I'd get 80+ headshots in a 100 kill match, my brother could launch grenades and land it ON the guy from any distance. All we needed was the knowledge that we had an hour free to play and we'd just relax into mindless fragging.
I've never really succeeded with the Zone like I have with the first Halo, possibly because I could perform every action at once, melee and shooting with the smallest of delay, I could estimate the exact height of a jump and compensate the aim perfectly. The first time I scored a sniper headshot while jumping I was like "Woo! Awesome!" but after a year it was a common occurrence.
Halo 2 wasn't as engaging, the firefights were over quick, no medpacks, slow melee.
Halo 3 is the same, though I was in the Zone all through Basic Training on Live, maybe because the map was so small I was constantly in the fight.
I find it much easier to enter the Zone with friends around, played online CoD4 with a friend sitting beside me with a 2nd 360, we both hit the zone, easily watching both screens working as a team without saying a thing scoring 20 each with only one death, attributed to one airstrike.
We did the same thing a week later, but I couldn't get in the Zone, I was too frustrated about a couple of early deaths, and I watched the friend get into the Zone and pull off amazingly timed "Frag-Stun-Fire" moves, thinking 5 steps ahead as he nades a window, stun through the door and then a split-second headshot while the guy is mid-way through falling prone.
I think it's basically when the front of your mind is attuned perfectly to the physics and mechanics of a game rather than the real world. If you can't get in the Zone try the following things:
- Headphones.
- Allocate a pure hour of gaming, and choose a game you know you can win.
- Ignore any setback.
- Take a packet of Shapes, pretzels, chips, anything you can stuff in your mouth quick. Plus a 1.5L of Coke, or sugary drink.
- Turn game sounds off and turn up music of your choice, high tempo for FPS, tense and/or epic for RTS.