In all honesty I think there really isn't anything else you can do?
You are doing everything I could possibly suggest. There are some regular products like this-
(btw, ignore the negative review on their.. some moron put oil in their dogs ears for some reason
Hmm. otherwise what kind of shampoo do you use? The brand can make all the difference in some cases. I like Earthbath.. and I use a nice oatmeal shampoo for my cats by I think it is OxyMed.