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-
http://www.amazon.com/Ark-Naturals-E.../dp/B0002AYR8O
(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.