The spectacle number depends on the curvature and length of eyeball. more or less these properties of our eye are determined genetically like our height. People with more than normal length of eyeball have minus numbers and shorter eyeballs have plus number. As our age increases, due to overall growth the eyeball length will also increase and so minus numbers tend to increase with age. As such it is not possible to reduce or halt progression of numbers. However some theories suggest that doing more near work induces myopia (minus numbers) Hence it can be tried logically to slow progression of minus number by engaging in more outdoor activities and less in indoor activities such as on tabs or computers etc.