I can remember experiencing Flow a few times in my life athletically. Sprinting one year in college, I was PR'ing in each heat, but felt like I wasn't even trying to run fast. I just felt relaxed. These days I will experience a part of flow at least once during the week. Show up to work and before you know it the day is done. That is flow as well.
The original book Flow by Ckiszentmihali, was a groundbreaking book. Start there if your interested in learning the science behind flow. I just recently finished The Rise of Superman, by Steven Kotler. Kotler is one of my favorite authors and I have read everything he has written. The Rise of Superman was filled with awesome stories and examples and techniques to potentially incorporate flow into your life. Great read, really enjoyable. There is a link at the bottom of the page.
He was recently on the Bulletproof podcast, #109 Steven Kotler and the Rise of Superman.