Cyl power of this much...a test should be done to rule out Keratoconous especially if it is changing rapidly ... There are no known methods to stop power from increasing . Also be sure if there is a medical requirement as in specific requirement of eye in the job to prevent future disappointment of getting rejected in Medical..

Usually eye power remains constant after age of 18 to 20 in most of the cases.  As the development stops eye power remains constant in most cases . That doesn't mean it doesn't increase later on , but less chances.  There is nothing you can do to stop it , but can get regular examinations to make sure everything is fine.  What do you mean by lot in your perspective.  An increase of 0.5 is an increase but not much . Also are you talking about spherical or cylindrical power.  If your cylindrical power is increasing it needs to be seen with caution . Needs additional investigations.