I think sumi otoshi, if well done by tori, is one of the techniques where you don't have a chance at all to direct your fall and try to get out of it via ushiro or mae ukemi. That works if tori does it slowly, gently or awkwardly (like myself).
We did sumi otoshi here in Istanbul (my vacation dojo) this Saturday, and mine was, as always, not efficient enough. So I asked the teacher to show me a) what was wrong with mine and b) what you could do to get full control over the throw. He showed me, and it was so swift and powerful that I went flying and made an enormous fall - there wouldn't have been a way NOT to take this ukemi, and it was far to quick to change its direction. What you always can do is try to buffer the fall with your free hand (turning it quickly over your back as Ron indicated; you can find that in You Tube looking for "tobu ukemi"), but that works only if you do it instinctively because you practiced it 1000 times before.
But then I think no one would ever throw you that way if he weren't pretty much sure that you CAN do that fall without danger.
Wish you much look - I always have all sorts of problems on the other side of the techniques: performing sumi otoshi or koshi nage on others....but at least it works on the receiving end