Hydrogen Peroxide 6% and isopropyl alcohol (rubbing alcohol). You can get these two at the pharmacy, usually at the "wound cleaning" section, as both are used to clean wounds.
Basically, I get a small bottle with a sharp tip, sort of like the vinegar bottles, and fill some water in. After that, I pour some alcohol in, and then add in the tea tree oil. Maybe a couple of drops is enough, depending on the size of your bottle. I don't add in the hydrogen peroxide until right before I want to use the cleaner, because mixing H202 with the water will make it oxidize.
Shake the bottle and pour into the dogs' ears, rub the base, and let him/her shake it out! That's it. Simple, isn't it? Be sure not to use this on ears with open sores or cuts because the alcohol will irritate it. I don't use much alcohol, it's just needed to help the ears dry naturally. Tea tree oil is supposed to help kill bacteria, and it smells pretty good, too, so I like it.
So that's it! Simple recipe. You can do a google search to check out other recipes. Most of them will include things like witch hazel, but I couldn't find that here, so had to skip it.