My eight year-old, female Shih Tzu (Lucy) has always balked at getting her teeth brushed, and it has gotten to the point where she refuses - no matter what treats or approach I take. Any advice?
I would advise a different approach: get her a raw bone, preferably of lamb or pork, since they do not break into sharp pieces and are safe to digest. Do not use beef as some have proven it could break their teeth since they are too hard; chicken or turkey bones do not break nicely and may be too soft to help her teeth; but depending on her size, a turkey drumstick might be great for her. That's how we started our puppy, with turkey drumsticks.
Raw bones are nature's toothbrush in a way. The action of gnawing on the bone helps reduce build up of bacteria. However, be sure not to give her cooked or smoked bones; they are very hard and could cause digestive problems.
It sounds to me that you may be better off taking a different approach alltogether. Have you considered dog treats specifically for teeth considerations, such as Greenies? Or how about dog toys for the same?
But for the purposes of teeth brushing, I'd try a different teeth-cleaning method, for example, if you are using a "toothbrush", try a finger brush. A different doggie toothpaste might also help. Keep praising and treating. You can try to build up confidence by taking little steps towards a bigger goal, such as treating when the teeth things are out, treating for the dog letting you put it in his mouth, treating for a few seconds of scrubbing, etc. Remember to praise and smile!