My advice includes behavioral techniques such as rewards for each step along the way to brushing (a token for opening wide; a token for each quadrant brushed; etc.); sensory work (allowing the child to chew the brush, gently brushing face and lips, trying an electric toothbrush); and offering the child a variety of toothpaste flavors (our son can't stand peppermint but loves strawberry). I'd also use social stories and visual cues with child who has mastered some language.
Are my ideas on the right track? What has worked for you?