Tonglen was introduced into Tibet in the 10th Century by an Indian named Atisha (Atiśa), who had learned this practice in Indonesia (Sumatra). Tonglen practice is most often is associated with a text by Chekawa Yeshe Dorje (1101-1175 A.D.) called "Seven Points of Mind Training," which includes some 59 slogans, pithy aphorisms intended to be meditated on. These slogans are very remarkable. The first time I read them I immediately felt that they were essential truths. You can read them here and see for yourself: http://en.wikipedia.org/wiki/Lojong