We use bribes lol. At that age we had major struggles too and had to pin him down but we're finally past that. I took him to the store with me and let him pick out a toothbrush (a spiderman one and a diego one) and then hyped him up about using it. We use colgate watermelon toothpaste so he likes the taste. DS is huge into books so we brush his teeth before his bedtime stories and tell him if he sits still and allows us to brush his teeth he will be allowed to come lie in our bed for some stories, but if he does not co-operate he will have to go straight to his own bed with no stories. He still complains a bit still, but he sits still for us.