Don't give angry team talks when you lose, keep trying to encourage your players. Post-match usually give plaise only for good performances (i usually say 'well-done' or 'pleased' only for a 7 or above, otherwise i say nothing).
Saying you're 'disappointed' or 'angry' post-match usually won't do anything, but make their morale even worse for the next game. I will use disappointed as a post match talk, only if their performance has been awful and their morale is high and they can handle it.
Avoid being too critical of players that get mad easily (you'll find out who they are as you go along), and the lower a player's morale is before a match, the more encouragement they need.
Other than that, winning seems to do the best job, but I have kept extremely good morale on losing teams, just by doing good team talks - which eventually leads to winning.