Re: Ukemi and Koshinage
Make sure that you actually get the slap down when you go over, since that is the actual "break fall". I spent an hour doing koshi nage about a month ago, and it took about thirty minutes before I finally started to get everything down.
Also, at least where I practice, it helps if uke stays straight like a board, so that when nage throws you you naturally flip over, your arm flies out and hits the mat first, then the rest of your body follows.
One last thing: look out for your *ahem* parts when you fall on your side. I think that's my last big problem with koshi nage breakfalls.