One thing he mentioned is that I have high shoulders, which is usually a bad thing, but he also said that it's possible that I just have a naturally high shoulder position. I'm not sure which is the case, but I can't seem to figure out how to lower my shoulders in any natural way (it always feels even more tense than in the high position).
There will always be deviations from the "ideal" in any person, because every person is different. Yet, the basic principles still hold true.
However, there is a huge trap, and one has to be aware of it. The trap is that one gets so accustomed to bad posture that the proper posture will feel unnatural or even painful. An example: Many people don't have the head aligned with the spine. It's often in front of the spine. Therefore, muscles in the neck and back need to work to stabilize the head. As a consequence, those muscles will always be contracted (tense), not matter what. If one tells them to relax, they will attain a state of minimum tension, but there will be tension nevertheless. When you now correct the posture by moving the head backwards a bit, those muscles will still be tense and will pull the head even more backwards, so that one has the feeling that the head will fall back. "That can't be proper posture" is what many people say in this situation. The trick is to slowly teach the body to adopt the new posture. It will take some time (weeks, months) for the muscles to learn to relax. They essentially have to rebuild themselves. So, the best way is to figure out if you indeed have a problem and then work on it slowly over time to correct it.
When I was trying to figure out what proper posture was, I looked up some information about Alexander Technique and how to teach it to yourself (I'm hate spending money). Most of what I saw seemed to be described so vaguely and with such eclectic terms that I felt like Alexander Technique was a cult of some sort. There were some "exercises" to try, none of which made any sense to me because I wasn't sure what it was I was supposed to feel.
- Saturn
I am not very familiar with the Alexander Technique. I do know that it does stress balanced posture, though. Friends who have studied the Alexander Technique tell me that it is impossible to do it on your own, because, as you say, you just don't know what to look for. I'd recommend a class to find out for yourself if it's the right thing for you. Ultimately, I think, one has to get a private teacher to get the greatest benefits. The best is to find a piano teacher who is, at the same time, a certified Alexander Technique teacher (or Taubman or Andover Technique, for that matter).
Two more comments about those "Techniques":
1. I don't think one needs to resort to fancy names for correct posture. Alexander, Taubman or Andover Technique, to me, seem to be based on common sense. They are not voodoo. As such, a good understanding of the human body is usually sufficient to get by very nicely.
2. Those techniques have initially been put together to treat injuries. My peef with many pianists is that they learn about injuries only when they already have them. Then it's often too late, or a lengthy process of re-training is required. Most people focus on learning to play the notes, fewer on learning to understand the music (the composer's life, harmonic structures, etc.). Even fewer actually look at the mechanics of a piano to understand the sound producing mechanisms. And finally, only very few take the time to try to understand the human body, its capabilities and limitations, although this might very well be the most important aspect of making music.
Comments, such as "I don't know what good posture is, but I can play Flight of the Bumblebee", or "I have this immense tension in my forearm, but I built up more strength, and now I can play this octave run." make me very sad as I can see these people having all kinds of serious problems later on. There is a reason why 86% of pianists have injuries. The reason is neglect of the human anatomy!