Sometimes things don't go according to plan, especially in the classroom. It is important to be prepared, so plan more activities than you think you'll need for your class. You may not get to all the activities, but you won't be scrambling for things to do to fill extra time. During class, keep your focus on the tasks at hand. Questions are a vital part of the learning process but don't take up time answering unnecessary or off-topic questions; you can address them later if time permits.
Writing out a list of things to accomplish during the lesson will help you stay on track. A small dry-erase board at the front of the room where all can see works well as a class agenda. A public agenda will allow students to see what the lesson entails and will answer the ever-popular question, "What are we doing today?"
Know how much information your students can handle at once. It is highly likely that your class is of mixed ability and not everyone learns the same way or at the same pace. If the students seem to not understand what you're teaching, it's OK to go back over the material they didn't absorb the first time. Conversely, if the students seem to grasp the concept faster than you expect, then it is fine to move on to the next task.
When engaging your class in partner or solo work be sure to set specific times for the activities so the students get a feel for how long the activity should take. For example, tell your students they have five minutes to complete a few questions. If they seem to be working hard and need more time, you can add some. It's easier to add time than it is to take it back.