How were photographers able to manual focus quickly back in the days? Aside from a lot of practice, was there a technique involved in shooting fast manual?
Some technique, yes. But of course, the focus screen wasn't the plain one on modern dSLRs - there was a split prism and microprism collar around it as focusing aids. You can get such focus screens for current bodies, too, although only some bodies feature easy user replacement of the screen (e.g. the 6D and 1D X do, the 5DIII does not).
I loved the focus screen on my OM-1.... split prism in the center..... makes me wish for the equivalent in the digital era.....
If you are manual focusing a DSLR you have to use live view. The viewfinder is just not good enough.... it gets you close, but not spot on. You really need live view and then to zoom in on your target to properly check focus. Easy to do on a tripod, but hand-held gets a lot trickier.
Also, and this might be my faulty memory speaking, but back in the good old days before auto-focus, when the lenses were designed for manual focus, there seemed to be a lot more twist to the focus mechanism than with today's auto-focus lenses. It used to be that you had around 180 degrees of twist to focus with, now it seems like less than 90, making it harder to precisely manual adjust a lens.