-ve power (diverging lenses) is to help the myopic eyes move the focal point further away to the correct desired position to give clear distant vision. So try not to use them while reading or straining your eyes while concentrating on something very close to your eyes while those glasses are on. This tends to get your eyes used to that  -ve power that you have on and the next thing you know is that you need more power. By not using them for the purpose that they are not supposed to be used for,,you can atleast have your power stop at where they are right now if not improve them.