I have a child with adhd, and have been through the whole tantrum thing. My son actually went through anger management therapy when he was around 5yrs old with a coginitive behavior psychologist. It helped him a lot. Then therapy again in 2nd grade for concentration and focus help.
For you, you already know to remain calm. The more angry I got, the more it fed into my child's tantrum, so I got good at looking calm. I was screaming inside, but being calm outside. The book, The Explosive Child by Greene is excellent for such children. Give it a read. Also, the Love and Logic parenting series is very good. It was created by a school principal and you may already be familiar with it.
It takes a lot of time, and it really wears a parent down, I know. We've done a lot of interventions here, Omega 3 fish oil, martial arts classes, and reading those books. Over time, it can get a lot better. You can see my son come to the edege now of a fit, but then pull himself back and calm down. Not everytime, but actually fairly often anymore. He is 11yrs now.
Oh, and I forogt, but what are you doing for yourself? I found it really good to exercise to help me be able to deal with my son. Exercise is not only good for the child with adhd, make sure he gets plenty, but also for the stressed paernt!