There are a number of things you can do to prevent or slow the progression of myopia.

Time outdoors

Being outside, for an optimal 2 hours a day, has been found to decrease the risk of myopia onset.  We used to think that outdoor time was only useful prior to one getting myopia. However, recentl studies have found that being outside, combined with exercise, can help slow the progression of myopia, even once it has found in the patient.

Diet

More and more diet is being looked at as a factor in myopia onset, especially a diet rich in refined carbohydrates.  This has an effect on insulin resistance and makes the sclera, the white part of the eye, more likely to lengthen.  

Near work

Another field being investigated is near work and myopia.  It seems, and makes sense that,  the closer that near work is done the more the effect on myopia. Also the longer we do near work the greater the effect on myopia progression.  Though, with the demands of modern life it may be hard to avoid the increase demands of near work placed on our eyes with education and many jobs requiring a lot of usage of computers.  The recommended working distance for reading, is what is known as the Harmon Distance, This is measured by placing a closed fist under the chin and then holding the book at the end of your elbow.