What's the best advice you've ever gotten from someone in the military?

Secure your gear.

As a young SGT I was pulled aside by my squad leader after making a correction on one of my Soldiers while deployed in 2004-2005. I guess I was a bit more stern and inconsiderate with my words than I thought I was. He told me "I've always raised my Soldiers the way I raised my kids". When I asked him to explain that he basically said he raised his children to be successful, disciplined, trustworthy, and hard workers. He used that same practice, albeit a few adjustments to compensate for the different environment, with his Soldiers and saw many of them gain success. That has stuck with me ever since and I took up that practice early on as an NCO. It has paid dividends long term. Soldiers I "raised" from 2005-2011 have sought me out or found me to say thanks when they've gained a level of success or been promoted. It's an exhilerating feeling and makes me proud and humble.

During Second Fallujah, I was talking with a Marine LtCol Battalion Commander outside a TOC during the ops pause.  I saw in the distance a young Marine having what appeared to be a major hissy fit, and I teased the CO about how "I thought all Marines were more stoic than that".   He just shrugged and said, "that Marine has had a bad day, which was the culmination of a bad week, which has been a part of a bad enlistment.  He's had to learn the hard what that every Marine loves the Corps, but the Corps doesn't love every Marine.  And that's true ultimately for every Marine".