Only little things to add to the wise words on here:
Don't answer a phone when you know you haven't got time for the conversation. People will only get huffy at being rushed off and if it's important, they'll ring again. It's only on TV that you can have a 3 sentence conversation and end the call!
Think twice before sharing confidences with others - even close friends. What's said can't be unsaid. Similarly don't believe that people are always after complete honesty when asking your opinion about some things: hair, clothes, weight etc. If someone's just had their hair cut and it doesn't suit them, for God's sake don't say so until it's grown out and then you can safely say 'Oh actually, it suits you better now than when it was short!'.
Treat everyone with respect until they give you cause not to...rather than the other way around. Some people think everyone has to earn respect, but I think we should be respectful as a default and only withdraw it if given cause.
Do the clearing up after a party before you go to bed! Sufficient unto the day is the mess thereof, or something. I've never regretted staying up another hour to sort out mess. It's actually restful and therapeutic and when you get up next day, you start with a clean slate.
Love the age you are. Honestly, as long as you have your health there is little to fear from getting older. You gain wisdom and confidence and if you go for style and elegance rather than chasing fashion and eternal youth, you'll look great.
Ask people about themselves (new people and old friends). They will love you for it! If you're naturally shy (like me) just think up a few questions in advance and even practise them. If you're shy, it's the best way to get other people to do the talking!