The Basics of Posture
Posture is the foundation of good playing. Unless you are playing in an ensemble, stand up when you play. Good flute posture will lead to much better sound. Of course, this does not apply if you are physically unable to stand. Stand as straight as possible with your head straight and flute parallel to the floor. Elbows should be out away from the body to allow free breathing and provide balance to the flute.
The flute should be supported by two points. First, the tip of the right thumb should support the flute between the index and middle fingers of the right hand. Second, the base of the index finger on the left hand should support the flute under the end of the rod near the neck of the flute. The remaining fingers should be curved closely over their respective keys.
Great care should be taken not to support the flute by pressing the lower lip against the embouchure plate as this will prevent flexibility of the lips and ruin sound. Likewise, the right thumb should not extend under or in front of the flute as this will inhibit movement of the right hand. Another common habit is to support the flute by raising the left shoulder. This may seem like a good idea but it has many undesired consequences. Raising the shoulder pulls the flutist’s mouth toward the shoulder and locks the lip against the flute. This limits flexibility of the embouchure.
Some musicians such as Vanessa-Mae, Yuja Wang, Andre Rieu and others have allowed the show to rise above the music (hey, I like a good show as much as the next person). It is a good idea to avoid dipping, swooping, bobbing, swaying and other forms of gesticulation while playing. It will hurt your music.
Practice Good Posture
The best way to develop good posture is to practice standing up with the music stand at face level. This presents a problem when performing however as most audiences do not to like to look at the back of a music stand with a flute sticking out the side. Once good posture has been established, it is good to practice at least some of the time with the stand in a lower position. In performance, musicians tend to keep their stand at around waste level so that the audience has full view of the players. This often leads to performers stooping forward or cocking their head to look askance at their music. Try a compromise position at chest level or memorize your music.
As with most rules, there are brilliant exceptions. Some of the greatest flutists in history have gotten away with things like slapping the keys flat fingered, tilting their head (and flute) 45 degrees or playing into their shoulder – not to mention Ian Anderson standing on one leg. Until your stardom is assured, I would advise doing it the right way.