Let your students know what your expectations are on the first day of class. Provide a classroom syllabus on the first day. This will not only make your expectations clear, but it will also make them in writing. Students should know right away whether they can eat or drink in class, what kinds of assignments to expect and how they will be graded. If you require students to sign the course syllabus, you can avoid the "I didn't know" excuse.
One way to increase attentiveness is to involve students in the material. Doing so can be as simple as asking students questions about the material you've have been covering, but it can also be more complex. For example, if the students were to have read a section of a play the night before, have volunteers read each character's part out loud. This gives students a chance to interact more with the material, and it gives the rest of the class some variety.
Class pacing is important for maintaining interest and limiting disruptions. Long class periods are more manageable if they are broken up into smaller segments. A three-hour class period isn't just taxing on the students, it's also taxing on the teacher. Break up that three-hour period into two or three segments to allow everyone to relax their minds a little. If possible, give the class a choice in how the class period should be broken up. Depending on your goals for the class, you could also mix up sessions of lecture with some opportunities for class discussions.
When behavior problems crop up, it is important to remain calm and collected. If the student is acting up on purpose, then getting angry will just encourage him to continue his behavior for the reaction. If the student isn't intentionally trying to get under your skin, then she will be easier to work with if you are respectful. If the behavior is actively disrupting the class, you should address it immediately by politely asking the student to stop. If the problem is something like poor attendance, try talking with the student privately before or after class to avoid causing him embarrassment.