I've been reading an article on military propaganda a few days ago. The term "propaganda" came from an early attempt to do so, the Catholic "propaganda de fe" or "Propagating the faith," hence the name. I note that there's a science part and an art part.
In the science part, you have to deliver the message, intact, to the intended recipient. And in wartime, you do not have access -- your target is usually behind the enemy lines where their government explicitly does not want you handing them fliers! So you have to somehow deliver it, intact, into your target's hands. No one reads destroyed, burned, or damaged leaflets.
On the art part, it has to be in your target's language, preferably with illustrations (because words alone are boring) (Yes, I realize the irony of that statement in a blog that has few pictures) and using your target's symbolisms. This is harder than it sounds. Cartoonists conventions like dialog balloons and thought bubbles are completely unknown in major segments of the world. Puns and wordplay almost never translate and have to be built in the target language from the ground up. Even heroes and villains vary greatly from group to group. PsyWarrior points out an embarrassing failure in an American attempt to influence Iraqis, in which a leaflet attempted to insult Saddam Hussein by comparing him to Hitler. Unfortunately for them, Iraqi knowledge of Hitler is that he was an anti-British and anti-Jewish leader, both of whom are seen as archenemies of Iraq. Whoops, it bolstered him instead.
The site, and others like it, go on to mention some general principles. The target of your propaganda is a hero, of course, or at worst a victim. Avoid playing into your enemy's hands. Try to sell your position. You won't succeed every time, but a good psychological operation wins battles without firing a single shot.
And bad psychological operations can lose a war. Case in point: Vietnam. Even a cursory glance into the propaganda shows that the American effort was scattered and disorganized, while the Vietnamese one managed to sow a continuous stream of fear, uncertainty, and doubt. American efforts were often demonstrably wrong, while the Vietnamese stuck to slightly more plausible claims. America won every battle and it didn't matter in the slightest.
Vietnam's still a touchy subject.
I'm in favor of Psychological warfare, because it leads to better and more productive peace agreements.