The embouchure is as personal as the flutist: a combination of physiology and training are the determining factors in developing the muscles used in flute playing. It helps to keep a magic 'space bubble' between the top lips and the teeth: anchor the top lip slightly on the canine teeth to allow a flexible, firm, but not overly tight top lip. The lower lip is the cushion over which the air flows, and the angle of both lips will change depending on the octave you need to execute. Above all, don't allow the corners to become upturned, in a smile, or overly tight. Use a mirror on your music stand to check in visually. Balance both sides of the mouth, even if the aperture is located slightly off of center. Harmonics and whistle tones are wonderful for developing correct embouchure habits.