How does he keep his voice inshape
Does he use something before studio takes

It's called "singing"

His voice rehearsals drive Keith mad

dadadadadadadadadaaa

lililililililililiii

dududududududududuuu

Iiiloveyou truuulyyy

And so on ;-)

Still amazes me how Mick could give permission to have that part released on Four Flicks...

Jelle

That's what I thought too Jelle. How Jagger allowed that to be seen. But Jagger I think is making an effort at times to be very forthcoming and show it all. In "being Mick" he revealed a lot too. So much more than he gives away in interviews.

"...no longer shall you trudge 'cross my peaceful mind."

I'm a singer myself and I can tell you that it's highly important to train the voice. It's even more important the older you get.
It's necessary to "warm up" before going on stage. Mick didn't do that on former tours - listen how he sounded sometimes. For singers the worst enemy is a cold or getting hoarse. Mick's just doing what his singing trainer has been telling him.