I have two children's mss I am shopping around right now. Story A has been sent to Publishers #1 - 6, while story B is at Publishers #7 - 12. When I get one rejection on A and one on B, I then flip them and send them to the opposite publisher. That way, each publisher has got one story by me at all times, but no more than one.
I'm hoping this works for the next few months. Once everyone rejects me, I'll figure out what I did wrong and start all over again (ok, in all fairness I've only gotten one rejection on each story so far)