I'm only 26 right now, but I figured I would share some of things I've learned so far that I find most important:

1. Maintain good credit and always pay your bills on time. Once ruined or lowered, credit is not an easy thing to fix. Don't max out your credit cards, your balance shouldn't be over 30% of your credit limit for that card. Don't close out old credit cards because the "older" your credit is, the better, and because your credit is young when you are in your 20s it is good to keep your credit looking as old as possible. Do research on it. Learn as much as you can.
2. Don't be afraid of the truth. Tell the truth and expect the truth, even when it hurts and it sucks. This will definitely make things easier and more straight forward in the long run. It applies to personal relationships as well as professional. If your boss asks you if you can finish in 2 weeks, but you know it'll take at least 4, don't tell your boss that you can finish in 2 just to make him/her happy. DON'T DO IT. BE HONEST. Tell them how long you think it'll take and why. They'll likely be more understand than you think while probably still expecting you to put in some extra hours to get it done in less time than you originally thought.
3. Learn to negotiate. Buying a house or car? Low ball the offer. Chances are the number they have posted is higher than they plan to get, meaning they've built their initial price expecting to be low balled or get more than the object is worth. Think you deserve more money at work, but the raise your boss presented you with isn't what you want? Say something. Tell them why you think you deserve more, but again be honest with yourself and with them. Do you really deserve more? Or are you just lying to yourself. Maybe they won't give you more even if you deserve it, but at least you tried. And now you need to decide what to do, which brings me to my next point.
4. Something not right in your life? Do something about it. No one else will. If you think you are being undervalued at work say something. If nothing comes from it look for new employment. Overweight? Work out. Feeling sad? Do something fun. Stop saying you'll do it tomorrow or later and DO IT NOW. Successful people do things when they need to be done, they grab life by the horns and do it.
5. Plan ahead. Think things through, don't plan out too many of the details, but think about the multiple possibilities and outcomes of a situation. Don't necessarily dwell on it, but give it some real thought.
6. Be adaptable. Going with the point above, you should plan ahead, but the reason I say don't plan out too many details is because NOTHING ever goes exactly to plan so you need to be ready to roll with the punches and take things as they come. Think on the fly, and don't be afraid to make decisions, which again segues into my next point.
7. Don't be afraid to fail. If you are afraid to fail then you'll never push yourself to do new and better things. Just be sure to learn from your failures and make the most of the lessons they provide. See a pretty girl or cute guy? Ask them out. Want to run a marathon but think to yourself "Well I can't even run a mile." It doesn't matter. TRY. Run the mile today. Tomorrow run 1.5, the next day run 2. You can do it. "Success is and continues to be defined as getting up one more time than you've been knocked down."
8. TRUST YOUR GUT. To me, this has been the most important. One major thing is though, learn to differentiate between your gut feelings and your emotions and animalistic instincts (AKA sexual needs). Your subconscious almost always knows what's right and what you should do. You need to learn to listen to it. It can get you out of many sticky situations as well as help you take advantage of great opportunities that may only arise for a second.
9. Don't stress out over the little things. Your room is a mess, your car needs to get washed, you have a paper to write, you have a deadline at work, you have dishes in the sink, your friends expect you to go out to dinner tonight, WHAT ARE YOU GOING TO DO?!?!! Simple. Write your paper. Finish your work. Tell your friends you'll catch up with them another time. Your dishes, your car, and your room can wait to be cleaned. When you're stressing about all the things you have to do, look at your list, and for each item ask yourself "What's the worst that could happen if I don't do this?" It will help you prioritize and realize that somethings can wait and they really aren't worth your mental health.
10. Be a realistic optimist. Keep your dreams in front of you and your head in the clouds, but always remember that your feet are on the ground and you're still just human. Have an idea for a start up? Awesome. Work at it, but remember, it might not work, it might not happen, and it will only happen if you work really hard, but it is possible. This goes hand in hand with number 7: Don't be afraid to fail. Always look at the upside of a situation, which also goes with number 9: Don't stress about the little things. Laugh. Laugh at everything. Everything is funny in some way. Find the good. Laugh a little, and know when to buckle down and be serious.