I believe that the best training you can do is with a full size air rifle, nothing will teach you the importance of technique, especially follow through better than a slug gun. The projectiles move so slowly through the barrel that the slightest error will make a big difference.
With centre fire rifles the important thing is not to develop a flinch, starting out too big is apparently the biggest cause of this.
I have seen some great suggestions here about fixing and testing for flinches. One good one I read was someone who had a mate load his rifle, at some point there was a snap cap loaded and the shooter could see himself flinch when he pulled the trigger and no bang resulted.
Another good one I read somewhere suggested just letting HEAPS of shots go without focusing on hitting a small target, just standing there pumping rounds into the embankment until you make the rifle your bitch.
Fixing a flinch would be very hard for people who have suffered injury's from rifles, A mate of mine handed a rifle to someone who said he knew what he was doing, before he could say STOP the bloke had the scope hard against his eye socket and let it rip...... lol James..... nice cut eye socket and shiner mate. I bet he would have a nasty flinch if he ever picked up a rifle again.
Your brain will eventually work out that no harm will come from the rifles recoil and loud bang with loads of practice.
Also look here
viewtopic.php?f=17&t=807 some very good suggestions there.