A chemistry teacher who is one of my customers took an interest in this and explained it to me. How i understand it, mixed bed resin like we use, is actually 2 different types of resin, one to exchange cations and one to exchange anions.
To regenerate the resin, you first need to seperate the two, which isnt easy.
Once seperate, to regenerate all you need to do is flush one resin with sulphric acid, and the other resin with a strong sodium hydroxide solution (caustic soda). Finally flush both with pure water and then remix them.
The difficult part is seperating the resins in the first place. But even then, the chemicals you use are quite nasty, will burn you if you spill them. As has been said, this is such a time consuming and expensive process, for the quantities we use its cheaper and easier just to buy new resin.