Pick your gear carefully, time it so that you buy a new marker or whatever when the price is down (after a new release for example) for a starter. In addition, any odd jobs you can find to do from washing windows to ironing, or even painting a shed can earn you some cash. It'll all add up, and it'll feel extra nice when you buy yourself that new shiny.
Just thinking about it as well, my old electronics teacher told me about a kid around your age who used to take cookies and the like into school that he'd bought just that morning from the shop. He bought a pack of 12 (or however many it was) for a few pounds, and sold the cookies at near enough 50p each. By the end of school and the few years of doing this the kid had bought himself a car that was more expensive than his parents.
Perhaps something like that could be worth a shot? It's amazing how much people will pay to spare themselves some effort.