The Urban Dictionary definition of "the zone" is really on the spot IMHO.
Being Completely unaware of what's going on around you as you are so extremely into what's going on right in front of your face; like anything more than 3 feet away just doesn't register in the brain.
It's really personal and here is what's working for me recently:
- A Spotify playlist of electronic music with few lyrics. I like this one from John Resig, the creator of jQuery, but there are many, just try some of them and find what you like
- Shut down notifications from your phone
- Use SelfControl or similar apps to block websites and social networks you use to procrastinate