The interesting thing about this mental state is that your personal goals, your training, your overall attitude and what you are striving for have a huge bearing on the outcome of a mushin event. In other words, forasmuch as you may react to a situation without thinking (we have all done this, more likely not in a martial arts sense - remember the last time you answered someone without thinking then had to go and apologise?
), the outcome will be determined by a lot of factors to do with you...
Taking an attack situation where you react instinctively, you might find (when the dust settles) that your reaction went too far (or not far enough!). This is not a coincidence - what comes out of you depends on what went in to you before you let yourself go.
This is why trying to achieve mushin should be preceded by a lot of meditation, introspection, cleansing - deconstruct yourself, examine the bits, identify what/who you think you want to be and be aware that this should be a living state (ie: you will always be changing, but you should try to guide those changes to "the better" with awareness), then put yourself back together with all that in mind, adding the bits you want and discarding those you don't want.
After that, when you slip into mushin, the reactions will result in an outcome that will probably be reasonably satisfactory (to you).
Apart from that, just train. A lot. And then some more.
Eventually, you will come to a state where you know that, when you let yourself go, you'll be where you want to be or thereabouts. There are no rules, and where you want to be may or may not be something I would be happy with, but it's not up to me: it's up to you. It _is_ you (a very pure expression of you). All I know is that the more I train, the more I seem to drift in the same direction as other people I know who have been training for a long, long time - we tend towards peace and harmony with a layer of steel in it. We blend and bend until it comes time to cut through - it is very difficult to blend or bend with confidence without having the ability & mental strength to cut through. That strength is the base that gives you the confidence to dial down your reactions. You have the ability to utterly destroy, but refrain from using that. A weaker base results in (often painful) escalation. It always irks me when someone who has also been training for a long time comes along and their expression clashes with what I percieve to be the majority. Then again, that's my problem, not theirs.
Sorry - I think I am in a rambling mood... Must be the holidays.