Arrange a signal at the beginning of the school year that means "stop what you're doing and pay attention to me." One that many teachers have found effective is a raised hand. Teach the children that when you raise your hand and hold it in that position, they need to sit down, give you their attention and raise both hands. Any students who are caught up in their own train of thought quickly become conspicuous and can be singled out. As an added benefit, their elevated hands can't be fidgeting with noisy things as you get their attention.
The enjoyment of games is behavioral bedrock among children this age, so use it to your advantage. Demanding silence is simple oppression from the child's perspective, but challenging them to remain silent for a set length of time, with prizes and incentives for those who do it best, makes it fun. Keep a supply of small rewards in your classroom, including toys, books and activities. Reward the best silence-keepers with periodic access to these rewards while the rest of the class is engaged in their regular work.
Find an LCD timer with a large display, one the kids can read from their desks. Explain to them that when they waste the time you need for teaching, you're going to take it back from them. When you find they're becoming unruly, ostentatiously start the timer. When they quiet down again, write the elapsed time on your chalkboard. Take that time away from an activity they enjoy such as recess or game time. You can achieve the same effect by standing up and pointing to the classroom clock rather than using a timer. Be exact with the times, so they know you're serious.
If you have an especially unsettled group in your class, divide them into four to six teams. Assign each team a name and organize them so that each group contains the same number of "difficult" students. Whenever you call for silence, rank each group in the order they became quiet. The official time is when the last child is seated and still. Praise the best performance and offer a small reward at the end of the day to members of that team. In most cases, peer pressure, team spirit and competitiveness will achieve the desired result.