The most common mistake made when brushing your teeth (in the two times a day you should be doing it) is with the angle in which you are putting the tooth brush.
When brushing your teeth, your toothbrush should be angled at a 45 degree angle, not at 90 degrees and the bristles are able to lay flat on the surface of the tooth. You want to make sure that you are grabbing all of the plaque from the tooth and moving it away, not just spreading it around.