The 'boatload' of money is how.
After a certain size, it is very difficult to have stable morale, something like 100+ soldiers or there abouts (it has been a while).
This fits quite well in historical terms, as soldiers didn't follow leaders (or at least not extended campaigns) out of loyalty or for their wages, it was more the possibility of loot! So the commander that can keep his army well stocked in loot will keep his army in the field. Hence the boat loads of money.
One way to deal with this in peace time or not when actively engaging in a fullscale assault is to garrison your men in a nearby town/castle, the way I usually do it is to keep about 90-100 low level troops in my party, so that they gain training experience, and then if I know I am going to attack a major army or town, I dump the recruits and load up with my max number of my best troops, take the objective, and dump them off again until they are needed for the next objective. This way my morale drops more slowly, or at least drops less of the time, because my negatives for party size are low or non-existent. Therefore inorder to keep my morale at a decent level I do not have to pay for a boost as often as if I constantly have my max army allowance, also by only 'holding' my low level troops, if some do desert it is no great loss, compared to losing elite heavy cavalry.