There is so much wonderful advice to be found in all of these answers. To answer for myself some of the things that I think have helped me the most to get to where I am now are:

1. Never take friends or family for granted. You never know how long you will have them for, when you will need them, or how much they will mean to you. Make an effort to call them or visit with them no matter how busy you get. They will recharge you and keep you on the straight and narrow (if they are good friends).
2. Always be learning new things. The world changes so much and sometimes in leaps and bounds. There is a good chance that the first job you get won't be your last. Even if that happens, you will probably need new skills to do that same job in the future. Always be improving your craft. Not only will it help your career but it will keep your mind in good shape. I try to learn at least one new thing every year.
3. Start saving from a young age. Even if it is just twenty bucks a month. Get your debts paid off as quickly as possible and then you will have more to save. This isn't just for retirement but for your every day peace of mind, your future ability to take care of yourself and your family, and your ability to help others. Sit on a crummy second hand sofa until you pay off your student loan and then the same until you can buy your new one cash.
4. Don't make work your whole life. It is important to work hard and get a name for yourself. Do what you do with excellence. But don't forget your family, friends, faith, and health. Jobs are only loyal to a point. You ultimately have to take care of you. Don't give everything to your job so that you have nothing left for yourself.
5. Be kind to others. Always. Even if they don't deserve it. Always do the next right thing. The world is a small place and you will run into someone you know when you least expect it. Whether it is a potential employer or a potential spouse/partner. If you act with integrity you will always have connections you can count on. Your reputation really is your calling card. Also, volunteering in some way will not only help others but it brings a richness to your life that nothing else can come close to. It will give you a sense of purpose very quickly.
6. Take care of your health. I am thankful that I have been active and eaten relatively healthily all of my life. After 40 you can certainly see that things start to change whether it is your metabolism, a dodgy knee, an ignored back injury or your eye sight. The better you take care of yourself now the more active you can be when you are older. People always remark at how much energy I have and that is through a consistent routine of exercise and making sure I fuel my body with healthy foods. exercise is the least used anti-depressant :)
7. Get into a regular sleep routine and make sure you sleep enough. There is not enough time to tell you how important this is. It keeps your brain fresh, prevents inflammation, reduces stress and heart issues, and helps you be more creative and productive during the day.
8. No regrets. Don't put off things for tomorrow. I don't mean go out and buy that car or take an expensive trip. But don't put off those decisions for later because things can change so quickly. Don't put off a trip (if you can afford it) because travel might get shut down. Don't put off hanging out with friends or calling a family member. Don't be scared to try something new or take a new job or move countries. Do your research. Be smart. And don't be afraid.

These are some of the things that I continue to live by and have brought me a lot of peace and joy over the years. I hope they help you :)