I use Cleanaural on mine about once a month and this seems to keep their ears clean and clear of mites. You can get a biggish bottle at a reasonable price from an online site like VetsUK and it last ages. Cup the ear flap to prevent liquid running all over the place, squirt in some of the drops, keep the flap cupped while you gently rub the base of the ear to encourage the drops to go down into the ear, then wipe out gently with cotton wool. Cotton buds can push any wax or gunge further into the ear and damage it, so shouldn't be used. The best preventive of ear mites, which like a warm, damp place to incubate, is getting fresh air into the ears - plenty of running around, and if your Cavalier's ears are very long and heavy, a little trimming and thinning helps.
Kate, Oliver and Aled