I tried re-using for the first time last time around. I flushed the soil out with plain water, let it dry out a little, flushed with florakleen, let it dry a little, then watered in hygrozyme. Then I topped it off a little bit with some new soil to replace what came out with the old rootballs.
I was nervous about how it was going to work out, but it turned out to be my best grow so far. From now on I'm going to just replace my soil every other time. (BTW I'm not organic).