This was a tough age for us too. My DS was very independent. The thing that saved us was variety, no single method worked for more than a week or so.
Ideas: electric toothbrush (but sometimes the vibration was too tickly for him),
different character toothbrushes (with these I would tell a story, for example, "Winnie-the-Pooh is looking for honey and wants to climb up the tree to look in the honey spot", i.e. his mouth, or "Dora is going on an adventure and wants to look for Swiper in the cave"),
playing dentist where he brushes my teeth then I brush his,
laying on my lap,
and other nights just letting him brush the best he could.