Deceased, huh? May it rest in peace.
Seriously, now, the key to high morale is getting a lot of varied food (the bonuses stack): You'll get more benefits from dried meat, fish, cheese, apples and bread than, say, a huge pile of just meat.
Secondly, you must win victories. The more enemies you defeat, the better -- conversely, if you lose or retreat, your party's morale will plummet.