1) Talk it out. Seriously, don't bottle everything up until it explodes and spews nasty mess all over the place. Confide in a close friend, parent, or just anyone who is willing to listen. Psychology studies say that it's better to talk out our problems than to try to attempt to patch up the entire world by ourselves.
2) Write it out. Take a scrap sheet of paper and write down everything that's been bugging you or anything that's on your mind at that moment. Write phrases, words, and even names of people all over the paper (yeah, I mean sideways, upside down, diagonally, and so forth) until the entire paper is covered...or until everything that's on your mind is out on paper. The important thing about this exercise is to keep writing and just let your mind jump freely from one thought to another. It's kinda like Sigmund Freud's technique of free association.
3) Try to make peace with those around you. Boyfriend/girlfriend problems? Talk it out--when I say that, I mean "face-to-face", NOT through text message. Always settle serious matters face-to-face. It'll save you a lot of misunderstandings in the long run.
4) Remember to take care of your basic needs. If you haven't eaten lunch today, go eat something. You haven't slept because you were up late working on a project? Go sleep. You have a killer headache and your limbs feel as if they're going to fall off? Take two Advil, get some rest, and see if you feel better in the morning.
5) Take a break from your stressor. Are you seriously stressed out because you have to perform a flute trio at Friday's student recital and you've had very limited rehearsal time because the trio has been working on two other flutes tunes, one of which is being cut due to time limitations and intense piano accompaniment and the other being a tune that is miserably long and hard and ridiculously fast for the first flute part which is...oh yeah, mine...and that also has to be recorded on DVD by the end of the week if we want to put that on the showcase performance...is that stressing you out? It sure is stressing me out. Here's some advice to you and to me: Stop worrying for a second, take a break from work (or in my case five hours of at home practice time) and chill for a sec.
6) Go find a nice quiet dark room and listen to your favorite song. Music oftentimes takes my mind off of things and it tends to be very healing for me. Sometimes when we're upset, we'll listen to music that fuels our dark moods. When people have bad days, they have a tendency to gravitate toward extremely depressing breakup songs or anger filled screamo/heavy metal music. Don't do that. Find your favorite song that's simply beautiful and just listen. Listen to the colors and textures of the music, close your eyes, and imagine.
7) Forgive and forget. Nobody is perfect. We're all going to make mistakes and we're all going to have shortcomings that we should just accept rather than dewell on them for all eternity. Forgive your friend for writing on the screen of your TI-84 graphing calculator with a mechanical pencil. Forgive your boyfriend for being completely clueless that you were indeed upset, hence the awkwardly silent and uncomfortable car ride home today. Forgive yourself for your own mistakes for not all of your mistakes--or any that I can possibly think of right now--will lead to the end of the world.
8) Breathe. If you feel like today was the worst day of your life because of one tiny little thing, just keep in mind that in five years from now you're not even going to remember this minor crisis. Don't make a mountain out of a molehill. In the words of a fellow flutist of mine, "If you have a bad audition, go home, watch some TV, and pig out on some chocolate ice cream. After that, move on and try again."
9) Keep in mind that tomorrow can and will be a better day for you if you can keep positive and not let anything bring you down.