I had tried everything with the problems ears of our dog and found that a product called Zymox worked the best. It comes in a form with or without hydrocortisone if that bothers you. It can be used it preventively also. I bought it over the internet two years ago and haven't used anything else since.

Product Description
Zymox combines 3 potent enzymes derived naturally from milk products. Each enzyme is unique and in combination provides a powerful anti-microbial defense system. The otic solution is highly effective against bacterial, fungal and yeast infections including staphyloccus, pseudomonas, proteus and malassezia. Zymox acts as a cleaner and medication with one daily dose. Do not use any other preparations during treatment.

Good luck.