tl;dr: The most joyful way to be alive.
I think “Living in the Moment” is a state of mind that we are blissfully not aware of when we’re really in it. If we know we’re in it, we’re really out of it, because then we are divided in thought, comparing thoughts about the past with thoughts about the present.
Living in the Moment is not about what we are doing at the moment but about how we are doing it. Whether it’s dancing, playing music or drawing a plan for a house to live in, we give it all we’ve got, with abandon, just for the heck of it, and forget about everything else.
We don’t dwell on that unpleasant argument we had this morning, or that important meeting we’re going to have this afternoon. We just do what the moment asks of us, light-heartedly, even if that means we are now defining goals for some future project not related to this moment.
It’s just work, and it happens to be the work of the moment. We are doing it “Without Desire for Results” (Bhagavad-gita). That’s also the way to get the best results because we are totally with it and not fighting ourselves at the same time.
It’s very simple and therefore not easy for an adult human being, but it’s definitely the most joyful way to be alive.