I guess this feeling doesn't came easy for everyone, and I think programming is not the easiest way to get this feeling. Playing a video game that you are totally into is the easiest way for me nowadays.
When I way a little kid, I could easily achieve this feeling by strong rhythmical body motion (swinging like a stereotypical autistic) and strongly focusing on an interesting object or image. I could "became" one with the object, and it was very satisfying. Without this flow/zone, I can't feel the object enough. Looking at it, holding it, etc is just not enough.
With a very bad analogy it's like desiring a woman, which can totally corrode your mind, but sex can relieve you from that feeling.
But that's getting a bit off now.
Anyway, it's worth to read through that wiki article, I'm sure you know this feeling if you think back on gaming sessions you had.
This brings back good memories of LAN parties with Q3 and CS, those were pretty much group flow.