If your cat is healthy, there is usually no need to clean his/her ears. I haven't needed to clean my kitties' ears in 2 years, though I do check them every week.
My Mozart did have a bad ear mite infestation when I adopted him from the shelter, and a treatment of Revolution, a combo flea treatment that's applied topically, cleared that right up without me having to clean his ears, which had a lot of brown gunk. I just swabbed what I could get to.
If you need to clean, then a little dab of mineral oil with a cotton ball. Other members will have better suggestions. Whatever you do, PLEASE DO NOT use QTips on their ears. Another board member did this for years, and thought it was fine, until her cat jerked one day, and she punctured his eardrum. It only takes the one time to do irreparable damage.