While as far as I know no clear details are given, in The Lord of the Rings Sauron seems to command huge numbers of humans as well as the Orcs — definitely the Haradrim and the Variags of Khand fight for him.
But how? Do those people know they are working for Sauron, and if so, why do they do it?
I suppose he could offer the local leaders power, but what could he actually offer them, and how would he keep control when they tried to run things for themselves? In the Second Age he presumably handled things through the kings wearing the Nine Rings, but at this point they're Nazgûl (creepy even when they don't actually have the supernatural fear "turned on") and thus not very good at ruling humans.
Sauron doesn't seem to travel around personally and demonstrate his supernatural powers.
The Orcs don't have any supernatural abilities to demonstrate 'divine sanction', are obviously unpleasant, and are individually weaker than humans.
So Sauron doesn't really seem to have an ability to 'show the flag' far from Mordor.
Does he just send the Nazgûl to scare leaders into doing what he wants? But there are only 9 of them and until well into LotR they have no special transportation — 9 guys really shouldn't be able to keep control of millions of square miles of land with pre-modern communications and transportation (and Sauron has only one palantír, so he can't use them as magical cell phones).