There are many good eye exercise for the short sighted persons. For example, you can go to the top of the mountain to look far away from the top side. This way helps you adjust the distance of pupils which may indirectly improve your eye vision. What's more, if you don't want to climb the mountain, you can do the eyes healthy protection exercise which is also very useful for the eyes.
It seems that you are really into baseball but your vison is not that satisfactory right? OK, considering your situation, I have some advice for you. First up, you should not have some bad habbits such as playing video games too much, reading in the bed, etc. This will help your vision stay at the current level. Next, you should get some consultancy from the eye doctor and see if you need some prescription glasses. Also, try to get some food good for your eyes and eye exercise is necessary.