I've posted this in other places around here, but I will post it here too.
A solution of 1/2 rubbing alcohol and 1/2 distilled vinegar is very good preventative care. It kills bacteria and helps dry the ear. If you have a dropper type bottle, just watch it out and do the mixture in that. Use about 3-6 drops per ear depending on the dog's size.
I know it works VERY well in animals AND humans alike. My daughter used to suffer from swimmer's ear constantly. The pediatrician recommended this mixture and she hasn't suffered since, and neither have the dogs since we use it every time they get wet, get bathed, or have their ears washed.