If nage is loading you onto his/her back and then dumping you off, they aren't doing koshinage properly. When nage is throwing like this, koshinage is always rougher on uke.
I would recommend turning your head toward nage and looking at him/her as you fall over his/her back. Do NOT look at the floor.
Feel for the mat after the manner of feeling for a wall in the dark instead of just blindly whipping your hand over and down toward the ground.
Ill try to keep the first point in mind, as I know Im guilty of that.
And good tip on the reaching part, as the slapping inevitably leads to my trying to catch myself with forearm and hand. (Typically catching the elbow.)
Thanks for the tips...