My Sensei often gets us to practice koshi nage breakfalls.
We practice by basicly throwing ourselves. Our partner bends over, we lie ourselves on his or her back and then allow ourselves to slide over their hips, one hand griping the underside of their gi (or hooking around their nearest arm) and the other sliding across their back so it knows when to slap.
Sometimes we will get the big blue crash mat out if there are any lower grades in the class.
I think the main points in falling from koshi safely (that differ from normal ukemi, the basics of ukemi should still be applied), are that nage should be aware and realise that by holding onto the correct arm they can raise one side of your body off the floor so you are always landing on your side and not on your back. But, uke should always be aware that nage may not know this and know that they can grabhold of the partners gi as they fall, which can have the same effect.
With practice none of that is necissary, but a great way to learn safely.
oh... whats O goshi naga?