I find with Cavaliers that it's worth cleaning their ears regularly. I do both of mine about once a month with Cleanaural. It also helps to clip the fur away from the actual ear opening (small sharp scissors do the job OK, but it helps to have an assistant!).

Kate, Oliver and Aled (who had terrible ear mites when he came into rescue, but has had clear ears since - plenty of running around helps to 'air' the ears!)