I don't know of any sources he used. This is one of those hard self-modifications that require highly developed emotional intelligence and introspection skills.

I know that when I tried to do something like that (not getting annoyed at a person for constantly bringing up the same settled point over and over for years), I failed. Basically, the feeling of annoyance flares up before I have a chance to consciously deconstruct it. I managed to quell it quicker, but not prevent it from happening. I tried preparing myself for the situation in advance, but that only made it worse, as I would get annoyed and upset during the simulation, as well. Actually alieving that a person close to you is basically a moist robot is hard.