Whether you as a teacher realize it or not, you are the best model of behavior in your classroom. A large part of your proactive behavior plans should include your own behavior you demonstrate to the students every day.
You must set expectations for your students, demonstrate the behaviors, and be vigilant to correct the kids. Don't waver on your expectations; inconsistencies will only confuse the students and cause you more problems.
If you stay calm, collected, and in control, your students will exhibit the same behaviors. The same is true about enthusiasm; if you are excited about your lesson and truly believe in its importance, the kids will respond in kind. Conversely, the kids will know when you are tired, bored, don't want to be there, or are 'winging it.'
If you are late to class, or don't start on time, the kids will pick up on it and be more likely to do the same. The same is true about the way you dress, the way you act, the language you use, and your 'body language'.
If you want your students working from 'coast to coast', or from bell to bell, you need to set the expectation of activity all hour. Start with a warm up, and ensure the kids are doing it. Keep them busy on activities with transitions between each. Don't let there be any down time. Work them to the end of the period, and have them pack up when you say so, not whenever they want to.
If you want your students to quietly read in class, but you are spending that time working on other things, it sends the message that you don't value the activity personally. Modeling the skill for the kids reinforces your belief that it is important. It shows you as a lifelong learner who values the skills you're teaching them.
The same is true for writing, or labs, or math problems. Students rarely have the chance to see real people performing schoolwork - for many, the only examples (and role models) are their classmates. Work along with your students.
Now this doesn't mean you have to do this the entire time. You must also supervise, coach, monitor, and actively support their learning. But you can spend at least a few minutes 'at their level'.