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Your score won't write itself... but here's some tips to get you started
Whether you are writing for a piano, full Orchestra or, and this has really been done, Vacuum cleaner, music has to start somewhere: The Idea. One of the worst things to happen when you are in the mood for composing is a total lack of ideas. However, if this does happen, and after a few minutes you draw a complete blank, some of these techniques might help.1 - The 5 minute keyboard bash
In this technique, you must make a recording of yourself playing the piano or keyboard (even if you can't play well, like the person writing this article) And listen back to it. You'll be surrised what you can find, some of the best ideas can turn up completely by accident.
2 - Give yourself rules
If you don't know where to begin, a useful jumpstart to your creativity is to set yourself some kind of system to work by. Examples of this common in modern classical music include the twelve tone system where tunes or harmonies are created by using all of the twelve notes but only once, and use of whole tone scales which eliminate semitones in the melody to give a most interesting sound!
3 - Make new arrangements
If there is a melody, chord sequence or tiny fragment of music tucked away in an old composition you wrote, or even a new one you feel had potential for development, take the very basics of that music, and try to move as far away as possible from the original concept and make it work as a new variation. If you wrote a slow, grim march, try arranging it as a quick, light waltz. Or a fugue. Or a children's TV theme tune.
4 - Sing
Yes - Sing. In the shower, down the road, in the car, sing whatever comes to mind. If something very creative comes to mind, sing it again, and over again. Sing it at work, learn to play it on an instrument if you can get to one... but don't miss the chance to get a good idea through singing, humming, whistling. This is much like a portable version of the first method, only this time, you really have to be onn the ball, and know a good idea when you hear one.
5 - Don't submit to plaigarism.
There is nothing worse than running out of ideas and so forming a songwriting duo with your photocopier. You stand a good chance of getting away with it, there being lots of material to choose from. You could just lift a tune off an old abstract composer, and nobody important enough will notice. But... don't. It's not satisfying, and it wont make you any better at composing.