Just to point out one thing the other answerers (especially F'x, Damien and Nicholas) haven't mentioned, do encourage and reward any contructive interruptions, like asking questions about the subject you're covering or, even better, answering questions asked by other students, pointing out any mistakes you may have made or letting you know if the material you're presenting has already been covered in another class.
One of the best things that could possibly happen, as far as engaging your students to learn is concerned, is having a spontaneous on-topic discussion between students emerge in your class. If there's any chance of that happening, you definitely should encourage it, even if it's cutting into time you'd planned to spend talking about something else. You can always make up for the lost time later.
Of course, the size of the class matters here. In a class of 10 people, you can just let the discussion unfold naturally; in a class of 200, you're going to have to hand out turns for speaking and make sure you don't let a discussion between a small group of students drag out so long that others get bored. Just try to do it without sounding dismissive.
How is all this relevant to maintaining discipline in class? Well, the thing is that the #1 cause of classroom misbehavior is boredom. (The #2 cause is probably the mistaken belief that you have to misbehave in class to be "cool".)
On one hand, the more your students get to engage in the teaching process and to guide it towards things they're interested in, the less bored they will be. On the other, if your students are getting bored, you'll want to know about it and find out why it's happening: Are you going too slow or too fast? Do they find the material you're presenting irrelevant? Or are they just too tired and unable to concentrate? The best (if not the only) way to know that is to encourage your students to provide you with honest feedback whenever they have trouble following your lecture or find it uninteresting.
Another trick that may help is to tell your students up front that attending classes is voluntary, as long as they understand that anything they miss will still be on the exam. If they don't want to stay in the classroom, it's better that they leave than get bored and distract others. That way, you'll get rid of the students who are bored because they already know the subject, as well as, hopefully, some of those who just feel like they have to misbehave. (The latter group may flunk the exam, but that will at least hopefully teach them a lesson for the next time.)
(If you do this, it's a good idea to post a detailed lesson plan in advance, and maybe make your lecture notes / slides available, so that students who choose not to attend can check if they'll miss, or have missed, anything they didn't know already.)
You should also encourage your students to come ask you after class if they feel like they're not keeping up or if there's something they just don't get. Not only does that give you a chance to help them over their stumbling block and to keep up with the class, but it also provides you with useful feedback on your teaching. If you find a lot of students getting stuck on a specific critical issue, you may even want to announce a change in your lesson plan and use the next lecture to focus on that point until you're pretty sure everybody gets it. Just make sure to keep asking for feedback as you do it.