I had a shih tzu many years ago, and yep, she had ear problems too. We repeatedly took her to the vet, but was told it wasn't mites. Part of her problem was that her ears weren't "breathing". They flopped down over themselves, and moisture would build up in them. It would get yucky, and she'd start scratching, making it that much worse. :(
The vet recommended a product to wash her ears out, that would also help dry the moisture. I can't remember what it was called, but it was a blue liquid in a clear plastic squeeze bottle.
The poor dog HATED having her ears washed, but it did help some (not 100%, though).