Be confident in your abilities. Model appropriate behavior. If you want the students to respect each other, respect them and their views. This will be a good way to gain their respect as they will see that you really believe in your own rules.
Be assertive, give eye contact, let your facial expression and tone of voice reinforce what you are saying, and maintain control and authority. Getting into an argument with a student (even if they are wrong) becomes a power struggle in which many students will be determined to win since they are in front of the peers. So you should have concrete and consistent consequences to apply immediately, and not worry about defending your practices with a student who may be challenging you and your authority.
The key to classroom discipline is consistency. One should be firm, but respectful. Students understand the necessity of rules, and that rules must be observed. They also understand the need for structure; and in fact actually desire it. One should not discipline students in such a way that the student considers it a personal attack, or that the teacher "doesn't like me." Children can be difficult at times, because they are first and last, children. It is thus important to not enforce a rule one time and ignore it the next; and also not to show undue emotion.
There is little to add to such good advice. Children really do want structure, and they do want rules and discipline in their classrooms. We need only remember our own childhoods and the games we played. How quickly did we set rules for our little games, and how upset were we when someone did not play by these rules?
I agree with #2. Fair and consistent are two really good guidelines for any teacher to start the year with. Here's an example. Our school does not allow gum-chewing, and I am unfortunately an expert spotter of gum and I spot it with some regularity. Once they get it, that I'm going to write a referral every single time no matter who and no matter when, they don't complain and they quit chewing gum in my presence. Neither of us has to say much, but the standard is maintained without any drama or whining. Set the bar high, be consistent and as fair as you know to be, and you'll be fine.
I've heard the phrase "don't smile until Christmas" before in establishing a classroom environment where you are in control. I think that's a bit drastic. A well-planned lesson each day with little down time will accomplish most of that, and as long as your expectations are reasonable, and clear to the students, and you stick to them, classroom management gets much easier.
In my experience, being firm in the classroom requires a balancing act. You need to let students know what you expect, and be fair in your expectations, but I believe you also need to be human. "Firm" denotes "solid," but in education, I think it must also mean "safe."
When students know what you expect, that you are fair and that you care for each one of them, they will learn that "firm" allows you to teach the class, but also to be supportive of them as they need it; often there are kids in your classroom that have no support and no structure at home to make them feel safe in their world. "Firm" does both: it's like "solid with a heart."
I think I would use the words fair and consistent to best sum up good discipline in any classroom. Students want to know what the rules are and that you will enforce them for everyone fairly. They also want to know what to expect from the teacher, and this applies to every aspect of the class, from instructional methods, to homework, to assessments, to discipline.