Shouting over the din of classroom noise may seem like a good idea, but then you become as loud as the students. This might be a signal to the whole classroom that they can be as loud as they want until your voice is louder, and therefore they'll feel there's a threshold they can reach without your interference. Sometimes you'll want absolute quiet, especially during tests. A good idea is to come up with fun hand cues, signals or body language that the children can understand. You can make it entertaining, with facial expressions, lights flickering on and off, or even use a little bell on the desk. Make sure the children understand what each of the signals means, whether they're supposed to be absolutely quiet or should just tone down the noise a little bit.
Children love to have fun in school. Elementary-aged children especially aren't quite at the level of understanding that they have to come to school to learn. They only see a lot of other children around them, and they just want to get to know each other. A good idea is to arrange an area in the back of the classroom where several items are available for playtime, such as games, puzzles, building toys, or even a computer desk. Assure the more disruptive children that if they behave, they can have scheduled time in this area toward the end of the day. Sometimes a little bribery doesn't hurt.
Children hate the word "don't." It's an instant invitation for them to "do." Instead of reinforcing your rules from a negative standpoint, it's beneficial to instill some positive influences. For example, instead of telling the children "don't chew gum in class", you could say "Please chew gum outside of the classroom", or change "Don't talk in class during tests" to "Please talk only when we're not having tests." Beginning each rule with the word "please" offers an extra boost of respect for you because you are trying in turn to respect the children around you, and they'll appreciate that.
Sometimes unruly children at the elementary level are bored and restless. They'll get into trouble because they feel like there's nothing else to do. You can win over your classroom by assigning tasks to some of the more troublesome students. Single out these feistier children and give them a few scheduled tasks, such as keeping an eye on the clock to remind you of something, or to check the floors for garbage upon leaving for the day. Designate another child as the person to call attendance. Giving some select children these basic responsibilities and rewarding them for their help, will make them feel more useful in the classroom and will influence them to pay you more attention and not be so disruptive.