I was wondering why sometimes it happens just like that without any effort, anyone has any experience on how to reach and maintain such a state?

Ah to be the still point that is also dancing.

Its a paradox. The moment we ask the question we are out of the moment. The moment we realize were in the experience we are tossed out of it. The moment we try is the moment we fail.

Their is not try only be. A exercise of Will that is a letting go of Will, a surrender of expectations, desire, thought.