What's the best advice on leadership you ever received from a leader?

I commend you on taking the initiative to seek knowledge on this. That alone already puts you ahead of your peers who aren't doing the same thing. Here in the Marine Corps we have an acronym called JJDIDTIEBUCKLE. It covers all leadership traits expected from any Marine in any leadership position; from the fireteam leader to the Commandant himself.
Justice
Judgement
Dependability
Integrity
Decisiveness
Tact
Initiative
Endurance
Bearing
Unselfishness
Courage
Knowledge
Loyalty
Enthusiasm
From my limited experience compared to those who commented here's what I have to say on each word here:
Justice: do not ever confuse this with personal vendetta.
Judgement: understand the situation, and the consequences of all possible actions.
Dependability: you're a team player. You don't have to be the star player or most-skilled. Pay attention to the tasks assigned to you. They are a pretty good indicator of how your command is willing to depend on you for something.
Integrity: do not ever violate the trust of others. You will likely never get it back. This applies not just here in the military but in life in general.
Decisiveness: for this I will give you advice I was given from a senior Corporal, "If you make a decision, follow it through. Even if it is a bad decision." The worst thing you can do, especially when lives are at stake, is make no decision at all.
Tact: again here is some advice from another Marine, this time a SSgt, "Tact is getting your point across without creating an enemy." Don't be disrespectful, but don't kiss ass either. It's a fine line that takes experience to find since the line is relative.
Initiative: don't be afraid to be the first to do something especially if it means bettering your team.
Endurance: as a leadership trait this has more to do with mental as opposed to physical endurance. Don't let bad things get you down. I usually don't have bad days at work, but when I do I tell myself at the end of the day that tomorrow is a new day; a chance to start fresh, and a chance to redeem myself assuming I messed up.
Bearing: goes along with mental endurance. The ability to keep professional when others don't, or have difficulty doing so.
Unselfishness: put others before you as long as doing so does not hurt you physically or mentally. Know the difference between needs versus wants. Putting others before you will pay off in the long run far better than any fruit salad on your ribbon bar. Always put your team's welfare first.
Courage: acting upon a difficult decision that may not necessarily benefit you, but benefits your team. Getting your team to work together is another form of courage as well.
Knowledge: know your job, know your teammates and leaders. Kind of going along the full cup saying: if you find your cup full, then take a drink so you have room in your cup again.
Loyalty: the obvious God (obviously not applicable if you don't believe in a higher being), country, branch of service, family, etc. Following all of the above will naturally show your loyalty to your teammates and your leaders. You should have a loyalty to yourself as well. In this aspect I mean your morals and ethics.
Enthusiasm: energy, both good and bad, is contagious. Don't acknowledge something sucks because that will make it click in one's mind that it sucks, and the experience will only feel worse. The mind's weird like that. Urge those who struggle in PT to keep going and finish strong. Encourage others to better themselves as you are currently seeking to do. Do your job the best you can, and then strive to do it better. Seek the advice from those with more experience on how to do a job better, or simply how to do it at all. Believe me they will notice.

PV2 Wilcox,
This is a great question. I met my first platoon sergeant, SFC Phillip Bottoms, in February 1985, during my first assignment in (then) West German. SFC Bottoms was the type of man you would easily forget - but would always remember: quiet, unassuming, thoughtful - and he truly cared for Soldiers. He shared his (what we would now consider to be a fairly simple) philosophy with me when I was a PFC, and I have claimed it, owned it, and shared it with every Soldier I have supervised since becoming an NCO back in September 1987:
1. Be where you are supposed to be.
2. Do what you are supposed to do.
3. Keep your supervisor informed.
Do those three things ... and you cannot fail. You will be a success.
A couple of years later, I would meet SFC Sherman Miller, who would counsel his soldiers on:
1. The first time is on me.
2. The second time is on you.
3. The third time, we have an issue ... which means YOU have a big issue.
I learned so much from these leaders. Among those lessons is to keep your messaging simple and something your troops will always remember. I have since developed a variety of 3-step messages designed to provide guidance and assistance for the success of my troops.
No two leaders are alike. No two leaders have the same messaging. No two leaders have the same approaches to success. Find what works for you - and share your knowledge and experience with those with whom you serve.
Best of luck.

The army is like a gumball machince full up color(which is different emotions) but when the army is mastered you see no color in the bubble gum and you've become a transparent leader.

"You'll have some good ones(leaders) and you'll have far more bad ones. You'll learn what to do and how to treat your people from both of the types. Don't f**k it up Noell, you're not a complete retard, as far as i can tell." This was told to me my first day as an E4 by one of the saltiest Chiefs(Navy E7) i ever met.

If you don't have anything for your soldiers to do, send them off... But tell them to keep their phones on.

When i first really became a leader i really didnt know what kind of leader i wanted to be, so i looked back at my not so effective leaders and knew what kind of leader i didnt want to be

I can get anybody to do anything by talking to them.
You only get fear and resentment by yelling at them
As in. We need to get this done, lets see what it will take. Good job.
Or
GO DO THIS NOW!

1) Strengths-Based Leadership
2) 7 Habits, Covey
3) Team of Teams
4) Leaders Eat Last

To mind my own business comes to mind for me.

A bad plan is better than a no plan...never give a bum a third chance.