Once I heard Srila Prabhupada say that one should concentrate on the sound of Hare Krishna Hare Krishna Krishna Krishna Hare Hare, Hare Rama Hare Rama Rama Rama Hare Hare to the exclusion of all other thoughts. On other occasions he said that one could be looking at a picture of Krishna while chanting, or chant while gazing at temple Deities. The main thrust I got from hearing what he said about chanting was that the mind should not be ‘elsewhere’ while one is chanting. So Srila Prabhupada was giving us some flexibility as to how we achieve this.