Get every patch your squadron sells. They tell your story unto themselves.
Take a photo every time you get to fly with a friend. Every time. (This photo is right after my favorite flight in a military airplane, and it was in an orange and white clown jet. I got to fly with one of my best friends. You couldn't buy this day from me for Jeff Bezos's entire fortune.)
Take every cross country you can. Take photos of the crew at the airplane, then turn the camera off for everything but scenic views.
Live where all of your peers live. Hunter's Cove in Kingsville was the best sense of community I've ever had in the military.
Use an online log book and keep it current for your civilian equivalencies. It will make things so much easier later on, even if you never want to go to the airlines.
When a buddy asks you to get a beer, lunch, dinner, golf, say yes, unless you're going to break crew rest or absolutely have something you have to focus on. But then follow up.
Don't neglect your family.
ORM out when you need to. Your life isn't worth it, and neither is your flight lead's. Everyone gets a "I can't fly, I was up all night on the toilet. I'm not sure I can even make the drive into work without a mess" card. It's universally understood. But don't abuse it.
The people. You'll miss the people like whoa.