Prussia and Austria have very different recruitment problems.
Austria has a lot of potetial manpower but lacks cash. Prussia is short of manpower but has plenty of cash.
As the Austrian player that tends to bias your recruitment towards line units and replacements (personally I avoid militia as they are pretty weak on the battlefield). With Prussia buy things that don't take much manpower (artillery eg) or is elite.
As Prussia you will be outnumbered but have a better army and far better leadership. Try never to trade losses at less than 1-2, as you simply run down your at start army against easily to replace regular Austrian units.
But, almost as in the opposite of that advice, you do have to inflict losses, especially in the first two years. If not the Austrians will simply recruit and build up a large replacement pool. Once they have that in place, you will end up losing the war as they can be beaten in the field and repair the damage pretty quickly.
So hit them as hard as you can in 1756-7, but really concentrate to do the sort of damage you need to inflict.