We always told the boys, use your left arm on the right side and the right arm on the left, brush one side until you can't lift up that arm then go to the otherside and brush, once you can't lift that arm go back and start over.

The brushing dry behind fans is a good way to time yourself, it will take about the same amount of time each day and the hair will get worked. Just try and start on the right side one day and the left the next, this way you make sure you are putting the same amount into both sides.