I don't think there was a day that went by in boot camp when I didn't quietly sing this to myself. On the last day before graduation, my senior drill instructor made me sing it for the entire platoon. I can't carry a tune in a bucket, but I belted it out with every fiber of my being, punishing the drill instructor more in 3 minutes than he did me in 3 months.
On my hardest days, something about this song still hits the spot.
https://www.youtube.com/watch?v=aRkvFO1sMco