I believe reaching the
perfect "mystical and elusive creature" is indeed impossible, but just as BajoranD said, you can still come very close, and that is enough.
From my own experience, you
can practice into the perfect mood. I've been trying to do it for a while now, with some small success. I'll try to explain my own method here - who knows if it'll work for anyone else.
Perhaps the most important thing to do is take your time. When you sit at the piano, don't touch the keyboard until you feel at ease. I usually take two steps here - first, I clear my mind. I don't move at all, I just... concentrate on not concentrating. (Heh, this
is difficult to describe!) Perhaps the closest way to put it is to listen to silence - concentrate as much as you can on hearing absolutely nothing. I get preoccupied doing this, and then I just stop listening at all, and then a good mood is achieved. It's basically a lack of thought, or emotion, or anything - just pure calm. Once I'm in this state, I concentrate on anything which holds a lot of meaning to me - a lot of the time, this will throw me back into a distracted state and I have to start over. But if it works, I start feeling that nice tingling sensation down my spine, and I place my hands on the piano, and pour all my thought into the music I'm about to play. If done right, I'll get very light-headed, and when I hit the first notes of a piece, it'll be pure music.
Though I've taken a lot of words to describe it, it usually takes me between 10 seconds and a minute to reach this state. When I'm practicing, though, I usually don't try to do this - it's somewhat mentally draining, and I like to reserve it for special occasions.
And of course, sometimes it just won't work. Usually, I can tell when I'm too distracted or fazed to even bother with this silence. It takes a good deal of self-control, and if you don't have any at the moment, it usually won't come flowing in from nowhere. This also takes a good deal of self-knowledge (yet another hazy term... this is really complex to explain!) - you have to understand what makes your own emotions free to the music.
Keeping this state during a piece is quite a good deal more difficult for me than attaining it to begin with. A lot of the time, I'll lose the music without even noticing, and when I realize I've phased out, I'll be unable to catch that state again. I really don't know a method to retain the music. If I did, I'd try my hand at explaining it.
I've also had a few random occurences of "absolute freedom." Most notably, I was playing a Bach Prelude somewhat routinely, just to refresh it in my fingers, and suddenly it hit me how beautiful the piece was. It was a rather powerful and sudden joy, enough that I nearly fell of the piano bench.
I also had a headache for the rest of the day, like when you're laughing too hard to the point where you feel giddy.
My most successful "absolute freedom" was at the biggest concert I've ever given. It was my senior recital, about 90 people came, some of whom I didn't know. When I sat at the piano and started playing, I was in a state of pure joy, which lasted almost until the entire concert. I lost it during the big finale piece, Rachmaninoff's Cello Sonata (just the first movement, but that's long enough to be a sonata by itself). I made a few nasty note mistakes, and that just threw me off completely. The rest of the piece was a struggle to stay in time and improvise well enough not to make too too many nasty-sounding noises in conjuction with the cellist.
Uhh, thanks for reading through all that if you did. I have a nasty habit of explaining things in as many words as possible.
Topic: Absolute freedom; truly achievable?