It depends on how dirty they are, usually I use a baby wipe, wrap it around my finger and then wipe out. If they are really dirty I use an ear cleanser from the Vet, squirt in their ears, massage the base, let them shake it out then wipe with a cotton ball or a baby wipe.
We've never needed to give a good clean as the pups have super clean ears so far. If they have "spots" of ickies we just wipe (or pick) it out. I like Sarah's olive oil idea though & will probably use that if we ever need a good, deep cleaning.