Find yourself an older child with a wobbly tooth, make sure that said tooth falls out in your toddler's presence and then tell toddler that their teeth will fall out like 'X' if they don't clean their teeth properly
Ds1's cousin's tooth fell out whilst we were having a picnic a couple of weeks ago - and ever since, DS1 has not fought having his teeth cleaned once - such a lovely change from having to pin him down to do it!!