Your horse is well trained at the stop, when you push down into the saddle and down into your heels and say 'whoa' the horse really plants itself, but time and time again, that horse stops on it's front end, you've done rollbacks and backing, well there is one little thing you may still be overlooking, does your horse have his head? Some horses are a little less physically gifted than others and need all the help they can get, in order for them to fully rock back onto their hinds and plant themselves for the stop, they need to be able to stretch their necks and often we don't trust them enough to just give them the rein, well we need to. Your horse will stop, and he will stop all the more smoothly, so plant your butt, say whoa and throw away those reins and see what happens! Please, use your common sense, if your horse isn't ready for this and still needs rein aids to stop, then by all means use them, this is just a pointer for perfecting those horses who are already well schooled at the stop. Now head outside, hop on that horse and stop! :)