My most productive time was about ten years ago when I discovered a café where I could spend my whole lunch time sitting in the corner with my little white ibook (and the café owner didn't mind). I actually finished my first short story in a week (time since for any story: about 3-8
years. Sigh).
That one hour a day was amazing.
However, I've never attained that level of concentration again. I'm constantly on the look out for a new café
