I have a handful of schnauzers that have that problem. I use benzoyl peroxide shampoo, card the back really good before clipping and leave the hair longer.
Saturday a schnauzer comes in that hasn't been in since Christmas. They took him to someone else in between. A couple of the bumps had become pretty infected and they looked awful.
I asked the owner about it and they told me that this happens every time they take the dog to someone other than me. So I guess what I do is working. The owners think I have the special touch. ha ha
"The most affectionate creature in the world is a wet dog." -Ambrose Bierce