Can we use yoga methods to develop more concentration in chanting?
Chanting the Hare Krishna mantra with attention is perhaps the single most important habit anyone interested in improving quality of consciousness can form. In principle, we should adopt any means necessary to always remember Krishna, and accept anything favorable for advancement in Krishna consciousness.
Srila Prabhupada sometimes drew parallels between chanting Hare Krishna and performing astanga yoga. For example, he would say that breath control is included within the chanting of the Hare Krishna mantra. This could be taken to mean that while chanting, one must breathe in between mantras. Such breathing is like yogic breathing because it directly provides the energy required for one to continue chanting.
However, he didn't instruct us to perform any form of yoga besides bhakti-yoga. He wisely understood the fickle and irresponsible mental condition of contemporary man. He made it clear in his books, lectures, conversations, and letters that chanting Hare Krishna alone can bring one all perfection, and no extraneous effort or adjunct yoga practice is required to attain our life's perfection.
This is not to say that we are forbidden to use our intelligence and employ whatever methods necessary to help us focus on our chanting. It is always wise to consult with more experienced practitioners when making adjustments to one's spiritual practices. With sincerity, intelligence, expert guidance, and a prayerful attitude, we can use anything in Krishna's service.