aside from artificial gravity, nothing. No amount of exercise bike pedaling will save your optic nerves from being in zero G too long.
There isn't really any good reason to put people in orbit for 6 months+. Rotate them out every couple of months. Yes we needed data on long-term microgravity effects on the human body. We have them now, zero G does bad things to your body. So don't do it for extended periods.
Fly in the ointment is the expected trip to Mars, which will take 9 months to a year. Fortunately people like Zubrin have developed advanced technologies to deal with this. It's called a rope. Attach the Mars spacecraft to a ballast via a rope (they call it tether) and spin it until you get 1/3rd G. Problem solved.