See Slate: Plan B: What to do when all else has failed to change your kid's behavior
...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. If you try something similar with your own recalcitrant child, within a few days he or she may well be using the toilet, eating green beans, or bathing without dire struggles.
