Twenty it is:
1. Put 1tbsp of butter in a sauce pan on medium-low heat, add 1tbsp flour to the melted butter, mix until you get a doughy consistency, and cook for a minute or two. Add 1cup of milk. Season lightly with pepper, parmesan, basil. Let it come to a boil, then immediately simmer it. It'll thicken up. You now have homemade Alfredo Sauce. Prepare to impress your friends, especially of the opposite sex.
2. Get involved in the things going on around you, even if it's just your schoolwork. Sitting in a dark room using a keyboard to talk to people a million miles away is not a social life. Talk to the people at class, even if it's just about the upcoming test. Join a club or something. Intramural sports are awesome. Varsity are good too if you've got it in you.
3. If it's meant to be, it'll happen. Don't rush it. It's nice to be a fiercely passionate person in all walks of life, but it's a lot less nice to be the guy who can't think ahead, or the girl who can't see the forest for the trees.
4. Slow and steady does not mean glacial. You're going to get spread pretty thin in life if you're doing it right. Learn to cut your losses on a project that's not going anywhere rather than wasting five minutes every two months on it. Better to come back to it refreshed.
5. Exercise really is important. We harp on it all the time, but if you seriously want to improve the way you FEEL about life: go to the gym. I come from a wrestling background, and Dan Gable is quoted as saying "Once you've wrestled, everything else in life is easy." Same thing with working out. If you go to the gym and work yourself out HARD a few times a week, you will come home and everything else will seem to fall into place.
6. They've all said it already, but it's got some truth to it. Disregard females. Acquire currency. It's nice to have someone who is a close friend. It's nice to have someone who will sleep with you. Do not make either a priority. If you treat people right and respect them, they'll be there in a few years when you're ACTUALLY an adult, and you guys can start making plans. However, you don't want to go out there wasting your time and money on somebody that's going to have giant life decisions to make in a few years, one of which will be "Has it gone as far as it's ever going to go?" Treat the opposite sex well, and feel free to spend time with them, but make it a fair deal, not a one-sided pursuit. Don't waste your time and money on them until you're ready to make a commitment to someone. (ps- At 20, you're not ready.)
7. Take every opportunity to travel. Broaden your horizons. See the world. If you're lucky, and aren't digging yourself into student debt, go on foreign-aid jobs during vacation periods. If you're like the rest of us, and need to make money, look into working abroad for a few years when you're done. Overseas experience is a HUGE boost on a job application. Many countries offer "working vacation" visas.
8. Always have fun. Work is hard. School is hard. Find something that's fun and keep doing it, no matter what else is going on in your life. Make time for it, or use it as a reward, but keep having fun. When your life becomes all work and no play, you become one of the drones helping to make this world a colder, more boring place.
9. Control your vices. Fun is fun, but too much fun is exactly that: too much. I like a drink. I set aside time and money to partake. It's not a lifelong commitment, but it's something I do to socialize with friends. I do not, however, fall down drunk four days a week. No one ever should. Once a week is plenty.
10. Milestones come and go. Woohoo! You're twenty! Big deal. You said it already: "it doesn't feel much different at all." The same is true with holidays, anniversaries and other celebrations. Too much stock is laid into arbitrary dates. Make every day count. Do things for a reason, not for a season.
11. The brands you wear are less important than the total package. If you're concerned about the way you look, it's better to spend time learning about Colour Theory than it is to figure out where you can find a good deal on designer phones/mp3players/computers/pants/shirts/cars. Buy for build quality, not perceived quality. Buy to last.
12. Your taste in <insert object> does not define who you are. You are not a collection of songs, movies and TV Shows. Be proud of what you like, but don't turn it into the definition of your personality (see: Goths, Trekkies, Metalheads.) Experience the multitude and be open to it all.
13. "Be the change you want to see in the world." Gandhi was a wise man, and this is probably the most important thing he ever said. No matter what you want from the world: be the exemplar rather than the fool crying for change. Lead by example, and preach from that example, but do not become the hypocrite who strives for a green planet while trashing his own house.
14. Respect yourself. Far too often in life do we question our instincts and decisions. If you're faced with unquestionable evidence that you've done something wrong, then accept it and move on. Until then, believe in who you are, and believe in what you do. You're right more often than you're wrong, even if you seem to be wrong an awful lot.
15. Get shit done. Work to completion. Finish what you start. Do everything within your power to make sure that the important things in life happen on time and with minimal worry. Procrastination is both a valuable stress-relief tool and a dangerous enemy. Use it wisely.
16. Learn the value of hard work. If you've never had a job: get one. ESPECIALLY if you don't need it. When I was twenty, this was one of the biggest lessons I had yet to learn. I still haven't learned the whole of it. Learn exactly what a dollar is worth to people. Learn how much it takes to earn one, and learn how much you can buy with one. Learn the lessons that money can't buy you at school. Learn punctuality and teamwork in a real environment.
17. Set your goals and achieve them. Think about it now. Where do you want to be when you're 25? 30? 40? Retired? Take some serious time and write out some serious goals for your personal life, your career, your fame and renown. If you want to be the billionaire madman with a harem and your face on the nightly news: plan it out. If your goals are much more humble: plan them out too. Even if you just want a nice, simple job with a wife and kids... start planning. Look at the road in front of you, or you're going to finish school/whatever and be left with no idea where to go from here.
18. It's okay to fail. Sometimes you are going to make the wrong decision. Accept it. Move on with your life. No one's perfect, they only pretend to be. Learn from your mistakes, but don't be afraid to make them. Someone's already said it above, but you learn more from what you do than what you don't do, and when you're old and dry, you're going to regret more the things you shied away from than the things you threw yourself into.
19. Don't argue on the internet. There are exceptions to the rule. It's one thing to make a strong argument. It's a whole other thing to be dragged into a drawn-out fight with an anonymous stranger. Avoid the latter. It's a waste of valuable time and you're going to gain what from it? Superiority? Be the better person to start with and walk away from the fool that wants to waste his time arguing trivialities.
20. Stop asking for advice on the internet. How much time have you spent, TODAY, watching this thread for updates? If you NEED advice, the internet is a great place to get diverse viewpoints. You didn't NEED advice today, though. Go outside and play.