good questions, I'm doing a current PBEM/AAR (link in my signature) and I'm going for the American reinforcements. Best way to do this is to occupy all the cities in one of the four regions.
Basically if you occupy no cities, they get continentals but few militia, if you occupy some cities they get a lot of reinforcements/replacements, as you occupy more they get militia but not continentals, if they hold no cities they get none.
So the worst is a few cities in each region.
I'm trying to clean up the far south in 1775, leaving enough in Boston to hold the city if I can (it is useful) while they can remain in supply. I'll then use the 1776 reinforcements to try and clean up another area (not sure which at the moment).
Not sure if it'll work but in an earlier game I got into horrible supply problems by concentrating in New England and around New York.
Another thing to look for is the sets of cities that generate extra EPs. As ever EPs are a key currency and can generate incredibly valuable supply carts, as well as force demobilisation on the Americans.
AJE The Hero, The Traitor and The Barbarian
PoN Manufacturing Italy; A clear bright sun
RoP The Mightiest Empires Fall
WIA Burning down the Houses; Wars in America; The Tea Wars