I don't know that I have a trick...if they won't do it, I will either do it for them or stand there and watch them do it and tell them they can't leave the bathroom until they do. Then again, my children are small so these things work.

I would just disengage on this issue and say something like, "I'd like you to brush your teeth so they don't rot out of your head and your breath doesn't stink, but im not going to stand over you and force you to do it. I will simply tell your father if you don't, and if he wants to address it, he can. I'm not going to ruin my morning fighting with you over this."

Lots of good suggestions in here, I just want to add one thought:

FIND a way to make them attend to their dental hygiene and do it properly and religiously twice a day!!! Seriously. More than anything else, including fluoride, brushing and flossing is what keeps teeth and gums strong and healthy. Judging by my (adult) skids AND husband, apparently if this habit doesn't get ingrained as a youngster they don't follow through as adults unless they have a reason ... I just loooovve reminding hubby and SD to brush their teeth *rolls eyes*