Śrīla Prabhupāda’s kirtan leading was always very meditative, deep, and focused. He would often end in a crescendo but it was steady, long, and unrushed.
“Gaura Gopala: I was right next to Prabhupada through the whole ceremony, playing the drum. He particularly liked to sing one tune through the whole time. He put his hands up in the air. He was dancing.” Ref. VedaBase => SPL 44: Let There Be a Temple
”It was clanging and banging. Kīrtana should be sweet and melodious.”
We had a kīrtana at the Rāma-līlā grounds, 1976 March, and Dīnanātha was leading, and tens
of thousands were attending and chanting. After the program Śrīla Prabhupāda and I were alone
in the back tent waiting for his servant and the car. As you know, he would often ask rhetorical
questions, and he asked me, “So, what did you think of the kīrtana?” Understanding this was
just a lead-in to his giving me an instruction, I answered with a bland “It was OK.”
Śrīla Prabhupāda’s definition to me then was as follows: “No, it was not nice. It was clanging
and banging. Kīrtana should be sweet and melodious. Come let us go to the ashram and have
kīrtana.” 
And so we went — Śrīla Prabhupāda, his servant, Baradrāj, and myself. Except for his servant,
the three of us sat in his room and Baradrāj played harmonium on the request of Śrīla
Prabhupāda, and we had a long kīrtana. . . . On Śrīla Prabhupāda’s signal, the kīrtana ended. He
looked at me, smiling, shaking his head a little, and said, “So . . . sweet and melodious.” And
then he moved on with the rest of preaching and hearing. I had heard him say, and heard that
he also said, sometimes stopping kīrtana, “No screaming and shouting.” [remembrance by
Tejiyas dāsa, 12 Nov 2002]
I personally feel that if one becomes more focused on the Holy Name of Krishna with love and worry less on changing the melody their leading of kirtan will become more and more ecstatic. This is especially in the case of those who are new to leading kirtan or who are not regular kirtan leaders. In their nervousness or anxiety, they change the melody, perhaps thinking that others are bored. This changing of melody may happen over 6 times in just a 15 minute period. (BTW I not talking about the different parts of a melody but rather changing to completely different melodies) Whereas kirtan stalwarts such as Indradyumna Swami, BB Govinda Swami, Madhava Prabhu and others keep it very steady and focused.
Once I noticed a devotee chant Hare Krishna for the 30 minute Sandhya Ārati for a total of 2 minutes. First he sang the standard Gaura Arati song and then the Hare Krishna Mantra for 2 minutes. (he also completely changed to different melodies within that 2 minute period) Then he stopped chanting the Hare Krishna Maha Mantra because he wanted to leave enough room for Jāya’ing at the end of the 
kirtan. Śrīla Prabhupāda never was Jāya’er himself, so it is not actually necessary to do a Jāya session at the end of kirtan. But the main thing is not to Jāya at the expensive of the Hare Krishna Maha Mantra. 
It is called the Maha Mantra for a reason, it is the greatest mantra. So my basic advice to new kirtan leaders is to keep steady the melody, even 
the parts of the melody should be steady, don’t shift it back and forth every few mantras and then run out tricks 2 minutes later.
In the past I have given advice to new kirtan leaders for the arati as follows. Chant the first part of the melody at least 10 times, then second at least 10 times. Then you can go back to the first part and pick up the tempo. The rest is up to you but starting if off like this is a good foundation.
So all the best to you. One simple item to test the efficacy of the one’s kirtan is to see if it creates smaraṇaṁ, even on a material level. Will those who have heard it have it dancing through their minds throughout the rest of the day? From śravaṇaṁ & kīrtanam comes smaraṇaṁ, rememberance.
BTW here is a link to the Harinama Sankirtan Handbook by Indradyumna Swami
