Without partiality for certain areas, mind training by itself should pervade everything, good or bad, which arises as an object of experience: other sentient beings, the four elements, or nonhuman beings. Deeply trained proficiency, not just lip service, is important.
From The Great Path of Awakening : An Easily Accessible Introduction for Ordinary People by Jamgon Kongtrul, translated by Ken McLeod. Copyright 1993 by Ken McLeod.
Published by arrangement with Shambhala Publications, Inc., Boston.
This site provides an on-line database of commentaries on the Tibetan Buddhist meditation practices of lojong (Mind Training) and tonglen.