When I had bare concrete, I used concrete cleaner and stain remover from Griot's Garage, followed by a pressure wash.
I had an epoxy floor put down a couple years ago, so all I've had to do now is some Simple Green and a scrub brush on the end of a long handle so I don't have to bend over. After that, hose it out and squeegee to help it dry.
This is what it looked like after the epoxy was applied and before I moved everything back in.
