1) Watch your matches, that gives an extra morale booster.
2) Make friendlies vs. weak inactive lower level managers and watch them too. With every win you get two boosters.
3) Use your blue boosters efficiently. With one booster you increase the morale only a bit. By using two you max the morale.
So wait until the morale is yellow or red, then use two boosters.
4) Buy new players with good morale that substitute your current ones with low morale.