The answer depends on the kind of people they are because I haven't experienced what a solider is like first hand.
I would be guessing like any person recognition for their efforts could boost morale because to be told that what you did was viewed as worthy and good is a pretty great feeling.
Another way would be to provide feedback. Personally when I receive feedback that is achievable that makes me want to work even harder to meet their expectations.
The last thing I suggest is letting them have fun at times when what they were doing is finished this kind of positive environment will not only boost their morale but also make them feel happy.