Kind of hard to give you a recipe book for success in NROTC, but generally speaking:
1) Show up on time/early - common mistake for underclassman who aren't used to the expectations of the program
2) Get involved - the bare minimum requirements for the program are very minimal, but showing your face around the unit for more than a couple hours a week (not including NS classes) will pay dividends later on when the staff is making BN billet decisions and even later when your CO's endorsement counts towards your service selection
3) Don't get into trouble out on campus. Not gonna say that I didn't know successful MIDN's who DID get into trouble with the law, but it certainly didn't make things easier on them, and truth be told, right now the Navy doesn't really need you that badly.....so like was said previously, don't f*** it up
4) Have a good attitude, and know your place. This will hold true beyond NROTC when you get out into the real Navy/USMC. Going along this, never pass up an opportunity to STFU and learn.
5) Don't think that being a good MIDN will save your butt from failing at school. When it comes down to it, the most important thing in the program is getting decent grades and getting that diploma so that they can pin bars on you after your senior year. I saw more than a couple MIDN f things away this way and get booted from the program.....guys who otherwise were solid players around the unit.
6) Don't be that guy/girl who is on remedial PT for like 2 years in a row, complaining about how its bad for everyone to be running on asphalt or that there wasn't a sufficient stretching warmup period. Everyone else somehow does just fine, so the only person you are fooling there is yourself.....and certainly not the PTO who has to drag you along on every PT session whining and complaining (I got the joy of doing this for several terms one year)
Good luck! It's a good program, and try to get everything you can out of your summer training/cruises. I learned more there than I did in 4.5 years back at school @ my unit.