Ear mites are something that you can very rarely get rid of COMPLETELY. They only get really bad when wax and dirt buildup in the ears gives them a good breeding ground. I think that as long as you are keeping their ears clean you shouldn't have much to worry about. On occasion after a cleaning, you can swab their ears with Mineral Oil. This is by far the most effective and cost-efficient means of waging war on the ear mites. Best of luck.
I use peroxide on a cotton swab. Mineral oil can be bought at any walmart etc. Most of the time if you clean their ears about once a week they should stay clear. Hardest is the dogs with floppy ears. Good luck