I definitely think exercise helps. Exercise, regular sleep, relaxation/meditation, and lots of red or green tea.

I didn't feel like exercising for quite a while after my worst concussion. In fact, I remember being stunned and confused as I tried to walk to school the day after the accident. By the time I got down the stairs and to the corner I felt like I was going to throw up. Gradually though, as I began to feel better (and this took several weeks), I started running again. Any sort of cardiovascular exercise is going to help your circulation and hence is probably good for your brain. I'm not so keen on any sort of movement or exercise that requires a lot of frenetic or sudden head movements, though.

Running is perfect for me, but of course you want to listen to what your own body tells you, and I'd wait a while before doing any kind of exercise after you've had an accident. Often times, the best I feel all day is right after one of my runs.
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About me:
I'll write an intro later, but in short I had moderately bad PCS for probably around 2 years after I was involved in a bike/car accident.