First, don't keep your eyes working for a long time. You'd better have a rest by looking in the long distance after you have studied an hour or so.
Second, pay attention to the following: Don't read in the sun or in a poor light; don't read in bed or in a moving bus. And when you read you should keep the book away from your eyes for about a foot.
And third, do eye-exercises every day. It will also help you keep good eyesight.
If you obey the rules above, I think, you may prevent your eyesight from becoming short-sighted.
