If you had a chance to speak with cadets and/or midshipmen before they commissioned, what wisdom, advice, and lessons would you offer them?

Give your troops something to brag about. Finish your qualifications early and well. Stay in shape. Give them every opportunity to teach you, and full credit when they help you succeed. Refuse any credit for THEIR successes, and take full responsibility for their failures. Refuse to eat before they eat or sleep before they sleep. Learn their home towns and the names of their wives and children. Take risks on their behalf. Routinely surprise the midnight watch with a hot cup of coffee, and know what they put in it. Never, ever lie to them.

Plenty of excellent advice already provided below. My only two cents for a Mid, Cadet or ROTC about to commission I guess would be 'take it one day at a time.' You're going to have good days and bad days. Don't give up on your career because of one bad day, one bad senior officer, one bad command.

Stick to the fundamentals and you'll be okay:
+ Work hard, pay attention, look genuinely interested (it'll be noticed at all levels);
+ Be early and try not to be the first guy down the gangway at the end of the day either (that'll also be noticed);
+ Respect people and their stuff, if you have the last cup of coffee, make a new pot. Pitch in if something has to get done, even if it's out of your lane.
+ Volunteer for additional duty / assignments (yup, it'll be noticed), but don't be afraid to ask for help if you're getting overwhelmed, and ask sooner than later. It can be phrased along the lines of 'excuse me Sir/Maam, but any advice on the best way to prioritize my Legal Officer extra duties versus my Division Officer daily stuff? I don't want to let anybody down.'
+ Always, always take care of your people, the men and women under your command, and not just work stuff, which is important, but as a military officer you're also now responsible for their physical, mental and financial well being (to varying degrees). You have to 'notice stuff,' like who's not getting work done, sleeping enough, going to medical/dental, and of course, signs of depression, etc. This is something your counterpart working in the office selling insurance does NOT have to worry about. If you have to go to bat for one of your people if you think they're being unfairly treated, then go to bat (because when they see you've got their back, then they'll have yours).

Now get out of here and go enjoy the best, most rewarding job you'll ever have in your life, whether you do 5 years or 30.

Don't show up with an attitude that you are in charge and will change the world. The men and women you lead or will lead will have an attitude towards you from the beginning until you prove yourself. Listen to your NCOs, they have time under their belt, the better ones want to get their mission and move out. Respect is earned, not given, take care of them and they will take care of you. You are part of a team even if it is just you. If you are prior enlisted, remember where you came from and what it was like.

Even though you have been an Enlisted member, use the knowledge of your Chiefs as they are there to mentor you in your JO role.
Take any job afforded you as you will establish yourself as a go getter and also help you gain knowledge of how things work.

Trust but verify. you will want to get your hands dirty, because that is what you did,even as a CPO. Don’t do the work for your troops, they will do just fine, but verify the know what they are doing and qualified. Risk Management! Ask questions, and provide praise often!

You have a new young officer to worry about. You! Stand strong and earn your trust in the Wardroom! It will go a long way when other officers that may not have your experience, trust you. Never “say, when I was enlisted or when I was station at…” You are a new officer and so is every future challenge. Look ahead, and never forget where you came from. Honor, Courage, Committment!

1. One of the strangest things in the military is the truism that you both lead your senior enlisted and must learn from them. As a senior officer, I would constantly remind my Chiefs Mess of their incredibly important role in training junior Officers.
2. Take care of your people. Know them, their goals and their families. Their goals may not be big enough and you may need to help them realize it. Give your people all the credit and keep all the blame for yourself.
3. Know your weapon system cold. Whether you are flying an aircraft, driving a ship, a tank, a submarine or leading a platoon, know the tactics and procedures better than the next guy.
4. Actively save for your retirement. Take advantage of TSP and save more than you earn. Bank half of every pay raise.

Take care of your people, everything else will take care of itself.

The new officers who use what they wear on their collar as a reason to be dismissive of their sailors’ experience or input very soon realize that they actually do need those sailors to accomplish things, but those sailors will all learn very quickly that that officer is not someone deserving of it.

Be appropriately professional, but actually work and speak with your sailors as people. Be approachable. Even before you’re competent at your role, by the fact that you’re an officer, you have easier access to the XO, CO, and CMC/COB than your Junior enlisted do. It doesn’t require any PQS, qual card, or warfare pin to be a good liaison for them if they need it for some reason (even something as simple as personally caring enough to make sure a leave chit isn’t sitting in a department heads inbox).

And definitely be consistent in your personality, demeanor, and judgment. Everyone hates a bipolar leader who they need to figure out if it’s the mean one or the nice one that day. They’d rather a consistent jerk than an unpredictable guy who’s chill sometimes.

Make friends with other LTs. A lot of good things can happen for platoons when the LTs in the BN are a united front. Moreover, support each other.
People will spend a lot of time giving you trash as soon as they see that bar, get thick skin and lean on each other.

As you learn from your NCOs, take time to get to know them as well. Don't get lost in the insurmountable mountain of work you will have infront of you and neglect to get to know your people. In the end, do your best and forget the rest.