Zymox Ear Cleanser is probably the best product out there, esp. for dogs prone to problems. It doesn't sting, and it helps break down bacteria and gunk. You can find it on Amazon, or through any good online pet supply. It's a watery liquid.
For healthy ears, I've sometimes used a 50/50 solution of apple cider vinegar (Bragg's, with the "mother") and filtered water (warmed to luke-warm). However, I'm leery of using that in problem-prone ears because it can sting (and once you have a dog experience that, ear cleaning will be tough in the future).
If you want an all-natural product formulated with essential oils, I like Wondercide's Natural Ear Wash too. They're a great little company, and this is also a watery (not oily) product. Their products always smell wonderful, and it's a non-stinging product. If you place an order with them, pick up one of their shampoo bars at the same time (it's life-changing, wonderful stuff that will cause you to never use bottle-shampoo again--the flea & tick one with citronella, geranium, and neem oil is excellent).