My lab was really bad for ear infections. A lot of it can be caused by water in the ear. Especially dirty water. You can get an ear drying/cleansing solution from most pet stores or the vet. I use it at the end of the day when I know he is done swimming. Especially if he has been in the marsh or in a river. In cleaner water his ears stay pink and clean no problem.
With him we also switched his food because. He was getting them even when I knew he hadn' t been swimming like late fall, early winter. The vet said it could be allergy related so we switched and I think it has made a difference.

If the infection is really bad you can get some antibiotic solutions from the vet. We have tried Otomax and Surolan and they both helped get rid of it in about a week.