Learn a physical anchoring technique to trigger the positive mental state you require. For example, every time you feel really good, click your fingers and concentrate on the good feeling. After a few practice sessions, whenever you click your fingers you will feel good again.
This is a simple explanation, Tony Robbins goes into into in more detail in some of his books.
Also, creating a cycle of heightened (goog) state - productivity - reward - heightened state again is a stable cycle that you could repeat from hour to hour to keep you in flow.
I think it is possible to make this heightened state the basic foundation of your personality although I have not achieved this yet in life.
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