There are lots of pedagogical articles advocating 'the right way' to sit and stand. It is always interesting to note that most players never consciously think about their posture in the creative moment: what is practiced every day just happens as a matter of muscle memory. When we teach, we need to remind students about ergonomics so that they learn how to sit/stand in the most efficient and relaxed manner  when they practice and perform. For  lesson and solo performances , as well as daily practice, standing is the preferred stance. Rather than focusing on which foot to place forward, I like to find a comfortable, tall balanced posture: relaxed and efficient. You must consider all the angles of the arms, head, neck, flute and torso. The legs and feet need to support the body weight. There are plenty of options for 'correct' posture that work, and lots of players change stance (shifting foot placement)  mid way thru a performance. When seated, it's all about finding the correct angle of arms, neck, head, flute, and torso, with the additional angle created at the hip joint.  Hyperextension of the lower back is not desired when sitting. Keep reminding your students about good posture at every lesson so that their bodies learn great position naturally without paying attention to dogmatic ideas. Consider Alexander Technique or Body Mapping to become more aware of these issues in your own playing. Use a mirror,  and encourage students to listen to their bodies so they can 'self correct'. 

There is an art to effective practice. Accomplishing goals will be assured with good planning.  A good teacher needs to help students learn how to practice at an age/skill appropriate level. For beginners, this may mean helping them remember to practice 5 times a week by keeping a practice chart. I like to require beginners to discuss their progress each week. It does not take long for students to see that the weeks where they make the most progress are the weeks where they were being the most consistent with their practice. Time management strategies are helpful for busy college students often juggling the demands of work and student life. Blocking out 4 separate one hour practices is as effective if not MORE so, than a single 4 hour practice room marathon session. Adult amateurs often face roadblocks to practice time because of the demands of their busy lives, or often a fear of failure, so helping them learn to love the creative time of practice is key. Progress = Practice! For each level, and for each student, there is a key to help them unlock their creative potential.