At first I was chanting Harinam on my beads, but I found that I was not getting the full benefit of the breathing. Then I switched to the Gopāla Mantra, and that was better. Today, I switched to Gopāla Mantra on the inward breath and Kāma Gāyatrī on the outward breath and found that it was full of possibilities and very powerful.
You breathe in with a small Mūla-bandha (contracting the sphincter and muscles and the base of the genitals), then visualize the mantra climbing up the spine with the inward breath.
oṁ śrīṁ klīṁ at the first chakra,
kṛṣṇāyaat the second,
govindāya at the third (navel),
gopī at the heart (appropriately),
jana at the throat,
vallabhāya between the eyes,
and svāhā in the thousand-petalled lotus.
Then start the outward breath extending outward but remaining in the cranium. When the breath is exhausted, one starts again with the same cycle. Since so'ham is the fundamental mantra of the inner and outer breath, saḥ = the Gopāla Mantra, and aham = Kāma Gāyatrī.
Although a relation between the Object (saḥ) and ourselves (aham) is present in both mantras, saḥ predominates in the first, and the verbs vidmahe, dhīmahi and pracodayāt all indicate types of relations between the sadhaka and the Supreme.
The parallels are:
kṛṣṇāya = kāmadevāya (vidmahe)
govindāya = puṣpa-bāṇāya (dhīmahi)
gopī-jana-vallabhāya = tan no'naṅgaḥ pracodayāt.
svāhā = (vidmahe, dhīmahi, pracodayāt)
Krishna is Kamadeva. This is the kaniṣṭha (pravarta) level of preliminary attraction. Krishna is the attractive force everywhere. He is the transcendental Cupid, behind the force of attraction in all things. Radha is the attracted. She is our Swamini and we are all part of her, as we are minor aspects of her energy. This we know (vidmahe), i.e., this is the essence of all sambandha-jñāna.
Govinda is Puṣpa-bāṇa. This is the madhyama stage, i.e., the sādhaka stage. Here the effects of Krishna's attraction are progressively felt in the five arrows of love. This we meditate on (dhīmahi), which is an appropriate indication of the abhidheya tattva.
Finally, Gopījanavallabha is anaṅga. This is the uttama stage. Here Krishna is manifest in the full līlā of the nitya-vihāra. Ananga also indicates the level of realization of Krishna's omnipresence, as described in the 11th canto of the Bhagavatam. Pracodayāt is, of course a prayer, that Ananga-deva inspire us to actually transcend physical desire and to enter the lila fully.