Children who have discovered they have some control in their lives and are resistive to their parent’s direction and instruction can be a more challenging issue. A term that extremely useful in these situations is "cheerful persistence."
First of all, parents need to be positive and keep a smile on their face when working with their child. Parents give many nonverbal cues to their children. If you go into a tooth-brushing session looking like you're going to war, more than likely it will be just that!
Secondly, oral hygiene is something that works only if it is undertaken on a regular basis. Therefore, daily brushing is a must. Avoiding tooth brushing because of the potential for a clash between a child and parent dramatically increases the potential for development of dental cavities.
Sources
http://www.aapd.org/hottopics/news.asp?NEWS_ID=1005
AAPD President Dr. William C. Berlocher (2009-2010): Quick Tips for Busy Parents: “I Don’t Wanna Brush!”