When the usual methods of getting your child to do something fail, perhaps try the exact opposite approach instead.
They direct the parents to temporarily back off almost entirely: to stop asking their child to do the desired behavior and say it's OK not to do it at all, stop offering praise or other rewards for doing it, and mask their attitude of engaged enthusiasm or frustrated rage with an appearance of bland disinterest in whether the child does it or not. What happens next, frequently, is that within a day or two the child starts doing the behavior with no prompting from parents or anyone else.
The explanation of why this technique works is pretty interesting. We've tried it a bit recently with Ollie and his extreme disinterest in brushing his teeth and we're seeing some promising results, although I imagine this works better with slightly older kids.