I would say mindfulness is perhaps the biggest empathy amplifier, but it is reading that has the biggest empathy growth impact for me.

Reading feeds my empathy by helping me to build and maintain a broad perspective; practising mindfulness helps me to apply it.

Many say that even reading pulp fiction can help to build empathy, so you probably need not read philosophers and economists if that's deterring to you; I can't personally testify to that though because fiction doesn't really hold my interest.